a couple things to consider:
mo already counts for an almost $7 million cap hit (his base salary this year), so an extension may not necessarily be that much of an incremental amount towards this year's cap. it could also include guaranteed monies next year and future bonus payments, or a signing bonus this year and a lower base salary which could even lower the 2015 cap charge. so a long-term extension wouldn't necessarily be all incremental cap charges, especially with him already being on the books for almost $7MM.
also, there are players counted against the cap who could be cut at a later date. guys like pace ($2.25MM) and babin ($1.625MM) may not make the team, and other than the recent $250K bonus payments they recently received, they can be cut with no cap impact. if we draft a pass rusher early in the draft, i'd guess at least one of them will be gone (if not both). then you have someone like darrin walls, who has a $1MM salary, who is no certainty to make the team with all the CB additions. so there is flexibility in the roster.