It'll be a dogfight next year again. The only two guaranteed to finish in the top 4 are Chelsea and City, purely because they can afford to spend buckets of money whenever they feel the need.
It'll be interesting to see who comes into United then who he brings in. Whoever comes in, it will take a while to gel a lot of new players into a team, especially playing against already strong and established team units like City, Chelsea, Liverpool, Arsenal, Everton who all understand how they are expected to play together.
The only thing in United's favor is having no European football.
Liverpool, Arsenal, City and Chelsea will all be expected to strengthen too with CL football coming.
Oh, and just BTW United went from 1967 to 1992 without a league title and that was without having to compete against mega-rich rivals but teams like Notts Forest, Ipswich, QPR, Derby, Leeds, Blackburn who all either won it or finished 2nd in that time.