I mostly agree with you.
The Kemper is a DSP engine at its roots for sure. IMO, the biggest value the platform has is its ability to very effectively parameterize a system response captured from a real amp so that it is very easy for a user to tweak after the profile to their own taste.
In theory, a PC (with the proper hardware) could do the same thing .... but they haven't. Kemper's IP isn't just that it captures the amp so well, it also:
1) Tweaks the amp so well
2) Provides an easy to understand and use EFX engine
3) Provides a cataloging system for performances
4) Provides hardware which is road worthy (A PC is not road worthy IMO)
5) Provides controls which tube amp users are used to dealing with (little or no training)
6) Provides a remote that is both road worthy, and well integrated into the KPA.
A VST won't do these things no matter how well they capture the amp system response. Additionally, the dedicated hardware in the Kemper provides a very low latency chain which allows all the processing to occur without phase and time alignment issues. While this is possible in the PC, it is really only seen IME on high end digital mixers.
Uhm -- for me, VSTs can include tweaking the amps quite well, but of course it depends on what we mean by that. Surely if external hardware is required for a particular barometer of "tweaking well", yea, VST will be subpar, considering what VSTs are. If however the standard would be tweaking that is more faithful to the amps, for example, that may be another story, depending on the VST -- but of course it's not like this has to do with something in particular about "being a VST" or "being hardware", considering what modelling units aim to do.
For me, the EQ controls on kemper don't get used much. Yes, they are easily accesible in a hardware format, but tonally tend not to do what I'm after. Usually it's a studio EQ that I'll use. That's convenient in kemper for sure, relatively speaking to some other solutions; but depending on the task at hand, for me, EQ corrections in daw can also be more convenient in other cases, some times going full-on VST anyway.
But in any case, I get what the main point is in terms of hardware vs software, even considering VSTs surely need some hardware to run (things being more about "some types of hardware vs other types of hardware" if I try to be as exact as I can).
If I was able to get the latency I wanted out of VST easily enough, and there was a portable unit running VSTs and offering few assignable controls, I'd probably use VSTs for way more than I do now. Usually hardware such as kemper offers a good mix of portability and other qualities I wont get with VST + PC. Other times, even kemper is too much to carry, and using VSTs is a god sent.
For tracking guitars ideally I much prefer hardware, kemper included, certainly, whenever possible. But if kemper was offered as a VST I know I'd certainly use it for quite a bit too, including for re-amping purposes, which would just be much faster that way. But what "hardware" unit offers would still be hard to let go.