It really depends on what kind of "telescope" you want.
For birding, you can be in situations where the key goal is identification. If that is the case you can get value out of a 300-400% crop or more. Even highly pixelated images at high magnifications can reveal field marks used for identification.
AF is a factor with birds - probably more than resolution in the range you are talking about. You can do a lot with a perfectly focused image even at 400% magnification, but if focus is missed it loses value pretty quickly. Focus accuracy can outweigh resolution for moving subjects.
If you are talking about distant fixed objects, the analysis is a little different. AF accuracy is still important, but you have Live View. For highly zoomed subjects there are even limits to the accuracy of Live View. And vibration is really a killer.
A spotting scope with adapter requires manual focus. It's probably better for scoping than for photos - but it does allow both.