The MCU is in a death spiral of making piss poor content (excepting Guardians 3 which was excellent, and Loki which was good). IMHO, it doesn’t matter what they do if they keep making bad content. And if they start making better content, the choice of whether to continue with Kang or not won’t matter much. I think alot of people are overthinking this very question.
However, in the interest of NOT ducking the question, I think what they’ve done with Kang to date is terrible. They’ve made him a disposable villain. Each movie can kill one of him because they’re an infinite number. Thus movie’s lose their stakes.
Worse, nothing he does really feels like a time traveller. Which is supposed to be his main schtick. I think they should have left the multiverse crap out, and had him seem unbeatable because everytime they beat him, he travels a little further into the past and ‘fixes’ his mistake. But time travel plots are tricky and you have to do them well or they come off as cheesy (see Flash, The).
At this point, they’d probably be better off transitioning to Fantastic Four and setting up Dr. Doom as the next big bad. Dr. Doom is WAY cooler than Kang anyway. The problem though is that they probably aren’t really ready for that. And see my issue in paragraph 1.