Inter's level wasn't sustainable, but I'd say that the fact they wouldn't/couldn't sustain it was almost by design.
Madrid was a nightmare job. Almost a lose-lose. And no forward planning possible.
Porto would have been successful however long he stayed.
Chelsea weren't run all that well back then. They were desperate for silverware, and just generally didn't operate that well. They're a much smarter club now.
This squad is shaping up to be his best ever (maybe slightly short of Madrid's squad under him - but these are all going to be players he's hand-picked.)
Chelsea of today is the best-run club he's worked for, and I think both he and Abramovich are in it for the long(er) run. The average age of his squad is also looking good.