Raposa it depends on the quality of the promoted team. Leeds, Leicester & Southampton all have better 11's this season than us, Burnley and Luton did last. Plus, it all depends on the promoted teams wanting to 'have a proper go'. Us & Luton didn't, Burnley had a muted 'go' but didn't recruit well enough.
I'd say Forest's spend of £300m is the benchmark for guaranteeing safety.
The PL is moving towards an american style model faster than you think (that's why they're buying up clubs), and i can see a huge remodeling of professional football in this country coming up. The PL will go to either no relegation or promotion or 1 up & 1 down, with the promoted team having to meet certain criteria on spend and stadium.
I also think that eventually the PL will tap up who they see as the next biggest 11 clubs in the country and break into a PL1 and PL 2 (including Celtic & Rangers). That will push some teams (possibly us & Wednesday) to merge and create a single franchise clubs.
The PL1 teams will remove themselves from the League Cup and play in European super leagues similar to CL, EL, and ECL. With PL2 remaining in domestic competitions. You may even see a global club cup competition.