Depends on the user.
Mine gets used on a 400mm prime and gives more than acceptable results for the few times I need that reach, so I saved over £5,000 by not buying the 800mm, as well as saving weight, buying a bigger heavier bag and having a far shorter, easier to handle setup.
The 70-200mm with 2x converter is nearly at the same level as the 100-400mm at 400mm, so could save a lot of people a lot of money. There's a good review
here that's worth a read.