There's a more serious (for them) element to their decline. In the recent investors call covering their accounts for the year to June 2019, Woodward said that not being in the CL this season would mean a loss of revenue for the current financial year. The estimated revenue for the 2020 financial year was in the range £560-580m. If they don't make the CL again next season they lose some commercial income. If they don't even make the EL that'll cost them even more, maybe taking them down to close to £500m for 2021.
The loss of that cash will hurt them badly & they will probably have to sell a couple of their better players to make up the shortfall. I doubt they will be short of candidates to desert the sinking ship if they aren't in Europe at all. The better players, if they get replaced at all, are replaced by cheaper players & the cycle of mediocrity continues.
We, on the other hand, should be reporting just over £560m for 2019, with the Puma deal plus the new PL TV deal kicking in for 2020. That's at least an extra £60m, giving us a revenue of £625m+ next year. That means we overtake them financially, by some distance. That's something I didn't think was ever going to be possible but it's going to happen in 12 months.