Changing fluids, transmission included, is a regular service for any car - or any machine for that matter. As for parts you can't replace... I hate to tell you that but that's sort of a staple of German cars. It's practically a running joke that you can buy an older M series BMW for peanuts, then have to sell an organ to get it in running order.
Look at cars like the GT86/BRZ - practically every part on that car has at least one 3rd party manufacturer replacement. In fact Toyota/Subaru literally have specification documents they released with the specifications for different parts of the car so 3rd party manufacturers could make customized parts. Same goes for the Mazda Roadster (Miata), Honda Civic, Toyota Hilux, Subaru Impreza/WRX STI, Lexus RF-C, Mitsubishi Lancer, etc. etc.. Having 3rd party parts available and long-going active communities around the car make maintenance a much more accessible task.
I've got an early model MR-S that's been treated like trash, thrown around, and generally treated poorly. I've only had to replace a few parts on it but all were purchased new out of parts catalogues almost 17 years after the car was made. And this is a car that is notorious for requiring a lot of maintenance for a Toyota. Definitely no headaches in the maintenance department for me.
If you haven't yet you should check out "Regular Car Reviews" on YouTube. It's hilarious but he also goes into a lot of history on cars, explains the reasons behind different defects and quirks, etc. No review for the older A8 but there is one for the A4 https://www.youtube.com/watch?...