That would require an increase in the size of the mirror, which would mean a complete redesign of the satellite. It is already at the limit of what can be observed with a mirror that size so a better sensor won't automatically give you better images.
If you look at professional cameras (Sony Alpha for example), the sensors improved in the last 10 years from taking shitty pictures at night to being able to take pictures in almost total darkness. All this with little improvements in optics.
So is the telescope sensor able to count each and every photon hitting it, was it already at maximum possible performance?
Yes, it can detect single far red shifted photons. It's cooled down to 7(!!) Kelvin to achieve that incredible feat, while the telescope itself is at a relatively balmy 36 Kelvin. If you want to get more spatial resolution you will need a (much) larger mirror and likely an even more stable platform. This is already a machine of such ridiculous precision that new technology had to be invented to make it possible, those sensors are works of art on par with anything ever made.
> If you want to get more spatial resolution you will need a (much) larger mirror
Or multiple mirrors spatially separated. Or a single mirror/camera at different positions at different times. Since JWST's orbit around L2 is fairly large, is that going to be used? You mentioned that stability of the platform would be a limit. How well is the position of the JWST known at any given time (I suppose it can be calibrated by viewing well known sources)?
That trick is hard enough when using radio telescopes connected to the mass of planet earth, with enormous wavelengths in comparison to what JWST is doing. It's position is known fairly accurately but even the telescope will undergo various vibrations from for instance the equipment on board and the course correction burns which will have a pretty significant effect on the satellite body.
The mirrors are dynamically deformed to correct for some errors, I don't think - but I also don't know for sure - if they are going to do long baseline tricks with the orbit, it would make good sense to do this for paralax shift measurements (which give you a good idea about the distance an object is at), but the orbit of the earth around the sun would be far more useful for that because it is so much larger.
Anyway, there are people on HN that are far more knowledgeable about this stuff than I am, I take it the designers and operators of the JWST are top in their fields and that anything interested laypeople can come up with has been debated, accepted or rejected a decade or more before this conversation, they're far from stupid, as evidenced by the incredible performance so far. Let's hope it stays that way and that the insertion burn goes well, that's the major scary thing that will happen next and the delta-v is nothing like the launch so I would assume that it will all go well but at the same time the telescope wasn't as fragile back then as it is now.
