I like ZX Next project. And it is 8 bit computer. But the 40 years of experience and know-how put into the design of those extensions make some kind of detailed comparison with real 80s 8bit computer unfair. Theoretically it would be maybe possible to produce machine like Next in ~1985-89 era, but I doubt it would be competitive on the market against 16bits and I guess the extensions wouldn't be as polished as they are today with the hindsight and knowledge of later systems used to design them.
So direct comparison with any original MSX model is a bit foolish, those computers are impressive in the context of time, and there's no need to sweat about having few less sprite pixels or a bit slower CPU. I have high respect for people working on MSX stuff.
That said, the new HW features of Next make the development for it for some type of people, like me, a bit more difficult. I always like to push the HW to the limit, and Next has kinda too many powerful features, making it really complicated machine to produce anything "on edge", requires lot more work than classic ZX.
On the other hand, it's perfect platform for people without any retro-dev experience, to get some entry point. Lot of the stuff which was very difficult on original ZX for newcomers (video ram layout, and 3.5MHz vs 28MHz, more memory) should make it easier to do some initial steps, and learn a bit about Z80 assembly. Also with the Internet and superior tools it's lot more easier today to learn about it.
Then again, back in the 90s without Internet, when I was stuck on some problem, I was stuck for days or even weeks, until I figured it out, but when I was rolling, there were no online distractions, so I was kinda lot more productive than today... 
My point is, that Next feels to me like extended ZX done "right" way, as it does work as regular ZX with 99.99% accuracy (and still being further improved in this regard), so I have working ZX, with a bonus of the Next included to have some fun with Z80 assembly today. But I would not compare it directly with classic MSX models and claim it is better or worse, it's just different world. Similar enough to get you taste of retro 8bit, if you never had MSX or ZX back in the day, but not equal enough to make any meaningful comparisons.
About porting Next games to more powerful MSX models. As long as the Next game doesn't push it to the limits, it should be mostly doable, but may get tricky. If somebody *wants* to, then why not, but I would not suggest or demand it, if somebody wants instead to create original new game for MSX, I guess that's even better (and maybe somebody will port it to Next later ).
About "MSX Next" idea - as non-MSX user - Next was tricky project, there were lot of smart and dedicated people involved, some even losing considerable amount of money in the project (hopefully to be recovered from second kickstarter), so I would advise to rather do that only if you have several very talented and dedicated people and if there's some actual market demand for such machine (if you want also newly designed case, you will need to produce batches in couple thousands to pay for the moulds and it will still cost fortune). Another advice is to pay homage to the original, ie. build any extensions in sensible way, how they hypothetically may have evolved on classic machines. Once you start slapping extensions on top of it without trying to mimic such continuity, the whole idea of machine will be IMHO difficult to defend, as you can just as well buy another PC or Pi4 or ... And have all that extra power and features. (for example slapping the WiFi ESP8266 module on Next turned out to be a bit of sore point, it's slowly coming to life with some SW produced and working around all those bugs and deficiencies in that chip, but that was nowhere as smooth and natural, as some other extensions of Next feel).
