My dad bought the Pioneer it was a mid of the table thing in the dvd era, so I'd say some 300 euros? For a complete 5.1 kit so I wasn't expecting real quality of course. And it does look like they made choices to restrict my ability to upgrade and eventually just get a new all-in-one kit.
Look, upgrading these was never an option. That's not how this kind of all-in-one product works. You buy it. You run it. When some part of it dies or you want something better, then you buy new.
If you want the option to upgrade then the kind of DVD/BD-based head unit & speaker kit you currently own is definitely not the type of product to buy. You buy something like this instead.
What you're looking for is an AV receiver with a speaker kit. You would then add a Blu-ray player or maybe a PS4 for games/DVD/BD playback.
AV receiver-based systems don't have to be from one manufacturer. You could have a Pioneer receiver with say a JBL Cinema 510 speaker package. The point is you get to pick the best matches to suit your needs and your budget.