I would argue that the PC is (or will be) a longer lived platform, with homogenized interfaces that let it stay in the game far better than the C64 could dream of. My budget 8 year old PC - which initially had the cheapest CPU I could possibly buy for the socket - is able to handle itself admirably well, even on newest generation games because of how far progressive PC upgrades can carry things these days. Meanwhile, I'm confident there is likely a complete chasm in what could be accomplished on a C64 with peripherals vs a cheap 386 PC Clone released just 8 years later (or an 1990 Amiga, or a Mac for that matter). And even a Mac of comparable price from the same era as my own PC would have been a shelf piece far earlier than this PC will be.

PCIe slots can carry you pretty darn far, and if you can use a Genesis cartridge, you can upgrade a PCI slot. For instance, on mine I added: USB 3.2, an NVMe slot, and a new graphics card. If I wanted to, I could throw in a faster network card. I could easily use the latest generation graphics cards without issue. I can play a game with at least 60fps in 4K. The only thing that really limits you is the CPU socket and the memory. Even then, after a few years you can throw in the highest end (or near highest end) server CPU for dirt cheap and some better and larger sticks RAM for not so bad a price either.