How so? You may be at a level where you gain more money, but players are also more expensive. Besides, you either spent resources improving your team, meaning you don't have that much left, or you didn't make that good on the competitions you played. With the improved team after 3 years of relegation, you would be able to do well on most competitions (perhaps excluding cup), so you would have gained less for a match, but go further in CL. And you wouldn't have to spend resources on new players because yours would do just fine, so you'd be storing all of them.
Now let's clear one thing. I don't tank for a living... I tanked last season because i was finding too hard to remain competitive due to token and booster shortage, and my team was loosing quality steadily. So i tanked a season, and have now tokens and booster for playing a few seasons at a good level.
One more thing. Even if you implement handicaps for the relegated teams, it is far too easy to tank and avoid relegation, so i see no point at it anyway. Like i said i tanked and finished 8th overall. Won many games while controlling not to get promoted.