The more I think about it, the more muddled the thinking is behind what Chelsea are doing here.
If Chelsea are really keen on building something for the long term, why get rid of Tuchel in the middle of a busy week, one week after the transfer window shuts? Why force yourself to do all that long term thinking in about 24 hours instead of over several months? Bringing in a manager where you're committing to at least 2 to 3 years no matter what happens requires serious due diligence. He may have potential, but Potter is a huge risk. If you not only bring in a manager like Potter, but build the club's senior management around him and it goes wrong, you are royally fecked (see United and Ole). So from a long term, empire building perspective, this is a terrible approach.
On the other hand, if the real concern is that this season is falling apart and you might miss out on top 4, why pick a manager with no track record whatsoever at this level? Potter is up against Pep, Klopp, ten Hag, Conte and Arteta, some real heavy hitters (plus Arteta). City apart, those other teams all have squads that are more or less comparable in quality. If it boils down to the ability of the coach to drag their teams through the season, you would have to put Potter flat last out of that group. That's not a reflection of his ceiling, which could be really high, its just a reflection of where he is right now, he's the manager of mid-table club. So from a short term, results focussed perspective, this is a terrible approach.
At this point in time my guess it that they'll go with Potter, make a big song and dance about it being a long term project but then ditch him at the first major wobble.