Inadvertent tanking lowers the stakes for new management, expectations get so low that they have a free path to making drastic changes, a "honeymoon" period. Players will accept anything, and the owners gave already seen sales plummet. And it only takes one or two deals to hit rock bottom (or often a fortuitous injury).
To tank a mediocre team takes a lot of work, you may have stars with NMCs, good young players that make it hard to be really bad, and so on. I mean you're trying to turn a 85-90 point team into a 50-60 point team, that's hard to do overnight. So 2-3 years tearing it down, 3-4 years in the bottom ten, then 2-3 years building it up to a SC contender. 7-10 years of futility, with no guarantee that you'll even garner a franchise player (Rangers got Lafreniere and Kukko with #1 and #2).
If all that pain doesn't give you a better shot than rebuilding without tanking, what's the point?
Flyers had two shots to "tank", after the Carter/Richards trade and after 2018-19.
By the time Briere took over, the team was firmly ensconced in mediocrity, too much talent to hit rock bottom, not enough quality veterans with expiring contracts to easily clear house (Rangers had a bunch of guys on their last season).
At this point, it makes sense to build on what they have, check back in three years, if it doesn't work, the new GM will have plenty of assets to add a new wave of young talent will taking a step back for a couple years (i.e trade the guys in their mid-20s for draft picks and guys in low 20s and draft high).