This should provoke a discussion and, of course, not all circumstances are the same.
For several years, my main camera has been a Canon 7DII. Professionally I do real estate and architectural work, some large musical groups on stage (100+ people), and whatever else might happen along. Personally, I have a longstanding love for landscape photography and a more recent interest in wildlife - particularly birds - that I find one of the most intriguing photographic challenges around. For this, the 7DII has been a generally good choice. For wildlife, I use a Canon 70-300L which has become my favorite lens. On the 7DII, it is usually long enough, the IQ is superb, and it's small enough to fit to go anywhere without extra effort.
Here's where the fun starts. This week, I will be ordering a new Canon R6 that I expect to mostly (but not entirely) replace the 7DII, which will become the backup. The old 50D will be relieved of backup duty and retired to a shelf in my office. But that presents a problem for the 70-300L. It will not be long enough to satisfy me on the R6 - although I expect it will become useful in ways it was not on the 7DII. Since the R6 has only a 20 MP sensor, I am wondering if using a 1.4 TC with it will be a better choice than cropping. The Canon TCs cannot be used with the 70-300L but the Kenkos can. I suspect that the hit to the IQ will be similar with heavy cropping or a TC, but saving some of those MPs may be important. I will probably get one of the new RF 800 f11s soon, but not right away, and don't anticipate being in a position to buy the new RF 100-500L very soon. The f4 500s (and longer) simply don't fit the way I work or the budget.
Sorry this is so long. If you made it this far, please comment. I'd like to see what others think. Thanks.