Regardles of the terms of the expansion process, given the NFL today, within 5 years you should be able to build a solid franchise between FA and the draft. The reason teams do poorly (or should do poorly) is when the need to take a year or 2 to clean up any cap issues, usually related to a bad signing or rookie who did not pan out (see MW).
The reality is the difference between the best and the worst teams are not talent of the players, because most teams have the same basic talent level. The difference comes from quality of depth, coaching, execution and luck (meaning health of the team, i.e. injuries).