Yes it would be awful tp spend a lot of money on a glass cage and then break it

Ultimately though it's the lack of a front door again or I would have been very tempted.
You are quite right about the plazas - they are indeed versatile and easy. And they do last a long time. Higgins has the older of the two that has been used and disinfected multiple times. I don't think the back needs reinforcing for a wheel generally, but the rodipet wheel is particularly heavy. I have had it in plazas before though without any issue.
The temptation of the square glass base of the Karat was also that it is deeper - no perspex needed - and I could use the rodipet wheel on its stand without losing floor space

But I bet bedding would still spill over the top of the glass base without perspex on the bars as the base is 25cm tall. It is also a very tall cage at 61 cm tall, so a bit too tall if you had 20cm of bedding in the 25cm base. It would be ok if you had 25 cm bedding but then it would fall out or need perspex anyway!
They never seem to get it right with these hybrid cages! ie clear base and barred top - which I think a lot of people would like. The Omlet gets that almost right - part clear base and still has a large front opening door on the bars.
Many years ago, I did look into getting a custom perspex base made for my Barney cage, but it was going to cost a fortune! And it still didn't have a front opening door!
The plazas may not have a clear straight square base, but they are very good to use for both hamster and human
I think the bars only rusted on this one as it was probably the one that was soaked in disinfectant after the moth outbreak - plus it was the oldest one. If the bars are even a little bit chewed in a particular area, they will go rusty after getting wet. There is the odd bit of rust on other bars on Higgins cage but I covered it with the perspex panels.
I think I will probably end up using the whole new cage and transferring everything across, because to just change the back panel, I'd have to take the whole top off anyway and remove all the shelves attached to the bars. It's a shame as it would smell a lot more familiar if I just changed the back panel and be less of a change for Higgins. I'm still thinking about it.
If I have to remove all the shelves (the big one is attached to the bars front and back) then I might replace the big shelf with two cloud platforms like Tino has (I do have two spare ones) as that would give a little more room in the cage. On the other hand it would probably mean losing the roof run. Higgins does love sitting in his sputnik! I need to work out whether it would be possible to keep the roof run still with the cloud platforms.
Ideally I'd like a 100cm plaza the same depth as the Omlet cage! But not as big as the Plaza 120. The Plaza 120 is lovely though and when set up it's great.