I'm not sure if I understand your question, but I'll try anyway. Of course I can't say for certain that playstation 4 cartridges won't fit in playstation 5, but it's a good guess. As technology progresses old cartridges won't have capacity enough to contain new games for new consoles and they may have to make different cartridges for the new consoles. See NES, SNES and N64 for reference. All produced by Nintendo, 3 different cartridges.
True, cartridges last longer than discs. Although this never happens to me anyway. The last disc of mine that got scratched to death is Tekken 2, and that's 12 or so years ago.1- My biggest issue with cd/dvd/blu-ray is that those things get ruined too easily (money wasted in the end).
the size of the games in 10 years will increase too though. they've definitely increased a lot since the beginning of this decade.3- Within a decade (probably far less), 50GB flash drives will be ass-cheap too.