The truth is teams are already doing this. Rosters should probably be expanded more (as we've discussed) for a variety of reasons but most teams are carrying a pitcher as the 26th man and it's semantics whether you call that last guy a sixth starter or a long reliever. (Ditto what the Phillies did with Strahm really). To some extent it's also semantics whether you plan for tandems/bullpen games, or just wait until they happen. And teams have always needed 8-9 starters. There just isn't enough good pitching, but it seems to me that will remain a problem whether you expect five guys to pitch 5-6 innings or expect 9 guys to pitch 3-4 innings.
In the end, it affects every team equally - except the ones with the best talent and best luck (and most money or minor league depth) will overcome it most easily.
The way the Phillies look right now a major pitching deadline move does feel inevitable. Someone better than Thor last year, and possibly sucking it up and paying the price for someone controllable (which would also be Nola insurance). They can probably get by without a starting-caliber position player but waiting on Painter is no longer an option (if he shows up and can still start or contribute bulk relief innings in September, great).
Moving Strahm back could also be an option but then you'd be putting an expiration date on his season for sure.