If that continues then it'd make sense that they intend to carry on building and expanding on what they've achieved rather than building it from the ground up every few years.
Except they wouldn't be, they would just be changing out the actors every once in a while.
And like I said, that'd be jarring. So far they got away with it since TIH wasn't all that successful and Norton's performance was so bland as to not leave much of a lasting impression and Cheadle played Rhodes as a much more interesting person, with seemingly better chemistry with Downey. Nobody noticed that Princess Diaries cosplayer in the Thor movies change actors because he was a one-note tertiary comic relief character and the second guy did a pretty good enough job imitating the other's actor's performance.
Plus that was all mostly early days for the MCU. Things are a lot different now and to just up and recast the likes Stark, or Romanov or Odinson and try to act like nothing happened would be neigh impossible without some serious backlash. Downey in particular owns the role of Stark to such a degree that they really can't recast the role. All they can do if he wants out is to find some way to retire the character and maybe find some legacy character to wear the armor.