All those long contracts we signed with players would have to be honoured or we sell. Disgruntled players wanting out. Less attractive for other player coming in. Less TV money. Not guaranteed to do well in that league so instead of "can't believe we lsot to Burnley", it'll be "Can't believe we lost to Barnsley".
Look at Leeds (never came back up), Wigan (League 1), Blackburn (League 1), Bolton, Sunderland, Birmingham, QPR (lower reaches of Championship), Norwich, Forest (never came back up), Derby, Villa.
Your username has 1988 in it, dunno if that is your birth year or when you started supporting Saints, if it is the latter then you may remember Luton Town and Oxford United. Where are they now?
No thanks.