a 2,750€ computer after 7 years no longer upgradable?

Some in the trade press have suggested that a new vulnerability similar to Meltdown hack has been discovered by researchers. Because the processors used in these Macs are extended versions of older processors, Intel has decided NOT to issue new firmware with protections agains these hacks, and Apple has taken that as an excuse to not offer Catalina on these processors.

Mac Pro silver towers are the only models that could run Mojave (with an updated graphics card) that are not officially supported for Catalina.

For your own personal use, there are some "install at own risk" hobbyist hacks that can install Catalina on those Macs, but we are discouraged from posting direct links because novice users could hurt themselves or damage their computers.

it's due to a limitation of the CPU architecture.