They've been boom and bust since Fergie left - basically trade off qualifying for CL every other year. This is their 11th season since he left, and they've finished top four 5 times, and finished outside it 6 times. Next season they'll have no European football at all.
I'm sure they'll be better when they bin off ETH but I don't see them bouncing back into CL football next season, or for the forseeable to be honest. It's surely only a matter of time before their commercial income starts sliding too - you can bank on being a legacy club for so long, but they've basically been shite for 10 years and look further away from the top than they ever have in that period.