Realistically, there were only a few actually practical options for a new phase/saga villain starting from where the MCU was in 2020.
Either you go bigger than Thanos, which means Kang and the multiverse or you go radically smaller than him with a built-in justification for why the Avengers can't just beat the bad guy.
The only real options they had for number 2 at the time were the skrulls/Secret Invasion or some flavor of political/scheming villain like Osbourne, Doom, Thunderbolt Ross, Valentina or Kingpin.
Osbourne would've been too risky because of the Sony deal, Ross or Valentina would've felt too small and kind off the cuff since that's not really who those characters are designed to be, and Kingpin would've been a massive departure from what people loved about the character on Netflix. Which means their best option was probably Doom, only they might have already been committed to a new direction before the Fox merger even went through. And it would've been weird to introduce Doom as an MCU wide villain for years before the F4 exist in the MCU.
Or there was secret option c of just not having an overarching villain for a while.