Unfortunately it's a bit shocking to read some of this discussion and I will try to push back on the sense I'm getting from it e.g. this focus on the NW operator is completely unjustifiable, or the apparent distinction between RQM and QFT in any serious sense, or the discussion about time operators.
First of all: the majority of string theory books/notes 'first quantize' a relativistic point particle. In these notes they promote time (not proper time) to an operator and arrive e.g. at the Klein-Gordon equation which tells us point particle wave functions are independent of proper time. See for example section 4.1 of
these notes.
If we trusted the discussion in this thread we would think every string theory book is incorrect on basic things like turning time into an operator in a relativistic context. Similarly every discussion of the quantized string promotes the time coordinate to an operator.
The deeper question is why in a relativistic context we can't impart the naive measurement interpretations to such operators - that of course is really related to the passage cited earlier, I would have come away thinking this is all incorrect if I took the earlier discussion of time operators at face value hence why I feel the need to push back.
The fact that Newton-Wigner does not treat time as an operator and is frame-dependent and breaks relativistic covariance is simply a complete embarrassment and is just one reason why this is a fringe topic nowadays.
The notion of a position operator for the position of a single particle at best depends on the interaction process being such that we can attach an interpretation to it that it did not create particle-anti-particle pairs and so completely change the system we were dealing with (if we were expecting a single particle remaining a single particle just interacting with some classical apparatus to hold at all times).
Why? Because of arguments such as that given in the quoted passage cited earlier, and the non-locality of position-space wave functions I mentioned earlier that people apparently also disagree with (despite this result being over 90 years old).
The claim
here for example is that the NW operator is only meaningful in such a case up to a certain minimal localization and can be used to help establish the validity of a one-particle theory, which, while useful, is at most small potatoes just like modelling particles moving in external potentials or something, which is also very useful, although it's even less useful and frame-dependent, hence the fringe-topic-ness.
The fact that a position operator is to a large extent useless in QFT (apart from the sense in the notes mentioned earlier, which is usually always bypassed and was in the early days) was known decades before Newton-Wigner (results they never even mention in their original paper), and it's application to things such as helping to determine the domain of validity of a one-particle theory as in the link above might be useful but it's irrelevant to the bigger picture, and really just has no relevance when talking about RQM vs QFT.
The sense one would get from this thread is that Newton-Wigner is a very important idea in QFT when it simply isn't, again why people should push back against such claims.
It is thus a very basic misunderstanding of QFT to think there is any more meaning to position measurements, QFT just fundamentally changes things: creating particle-antiparticle pairs is inevitable so the idea of measuring e.g. a single particle using a measuring apparatus modeled as one or a fixed number of particles is simply the wrong way to think about things.
The main point is: relativistic quantum mechanics is quantum field theory, there is no essential difference between them except for some of the formal tools we use to derive results, in other words 1st vs 2nd quantization, just as there is no essential difference other than the tools we use when doing first vs second quantized non-relativistic quantum mechanics. One can go back and forth between these formalisms at will in principle, indeed nothing would make sense if one couldn't, it would be a huge flaw in quantum theory if this wasn't the case.
If this is shocking, one just needs to ask oneself: why is it that we can go back and forth between first and second quantization in NRQM but not in RQM? The answer is obvious that we can in both.
There are plenty of arguments justifying all of this for example in the textbook that the earlier quote was taken from if people are interested, none of this is really debatable stuff to be honest, none of it is thrown into question/doubt especially not due to the apparent 'importance' of the NW operator...
To address the reason people think RQM and QFT they are distinct.
The real problem/issue here is that people apparently think that when we do relativistic quantum mechanics it somehow implies we are forced to work with one or a finite number of particles which would mean viewing RQM as an almost exact copy of elementary NRQM problems up to choices of potentials etc... (and so we can talk about position operators for such systems, despite the critical flaw of ignoring that interactions create particle-antiparticle pairs thus changing the number of particles one needs to include to accurately describe the system, simply a fatal flaw to ignore...).
But it is simply a completely unjustifiable assumption to expect the number of particles to remain fixed in a relativistic theory. This unavoidable fact has to be accounted for when doing RQM in a non-2nd quantized form. Furthermore the finite speed of light (RQM) vs an infinite speed of light (NRQM) has to be accounted for in the uncertainty principle which leads to such serious implications that it even changes our interpretation of the wave function. Even worse, time and space have to be treated on an equal footing, which is why the string theory notes cited above do exactly this.
The fact that we don't attribute the naive meaning to these time-space operators, and don't demand such things reproduce NW operators or other things, is to do with the quoted L&L passage that people are disagreeing with - one would be completely lost when seeing quantized time operators in a QFT context otherwise so one should just trust good textbooks more than discussion forums.
The reason QFT is so useful is that in one fell swoop we can deal with any number of identical particles as part of one big 'field' - it's not mandatory that we take this 2nd quantized view but everything becomes clear/easy (e.g. a 'number operator' and it not being conserved is obvious) when one does. It especially clears up the interpretation of the 'negative energy' eigenfunctions for example.