30-40 years is probably about right for the PL in fact it is possibly fair to say they may never win it again in its current format. I know the whole Super League thing came and went in a mad, crazy week - but I'm pretty sure that in the next few decades there will be a move to a proper European Super League, to replace the national competitions not co-exist alongside them. The only thing perhaps stopping that would be the drive on climate change, as it would involve so much international travel week in and week out.
Winning the CL is more attainable, as Liverpool did in their fallow years.
Where Utd. have a problem is financially: Liverpool are now very close to Utd. in terms of financial muscle and neither can compete with Chelsea, Man City and now Newcastle Utd. So consequently a Premier League for either Man Utd. or indeed Liverpool (if they don't win this year) could be many years in the waiting, due to the clubs backed by states with almost limitless spending.
Its a shame the Financial Fair Play rules have become pretty pointless. A bit like how massive Companies (Amazon/Starbucks etc.) avoid tax, or massively rich people pay clever accountants to limit their tax, these super-rich clubs can pay clever people to find loop-holes in ffp which was supposed to create a more level playing field, but sadly, it has not.