Only Adam won't even be 33 until the end of next season, is a demonstrably better kicker than say Brown (who only made 72% of his attempts this season and averages about the same kickoff distance) or even Longwell (whose % is off this year and whose career kickoffs are a tad shorter on average). Longwell would cost as muchas Adam, while Brown, at 27, would potentially save about a million and a half this season, but cost a draft pick in RFA compensation and be a UFA next season (with a franchise tag approaching $2M), not to mention he is far less accurate on the road (64% vs. 86%) in the cold or close and late. Penny wise and pound foolish.
Vinatieri is the most consistently bankable available kicker in the league for the next 4-5 years. He's never missed a start, works out like a football player, and he's clutch (something Longwell hasn't had much opportunity to prove or disprove lately and Brown hasn't been around long enough to have even been tested) and he keeps his mouth shut in public where contracts are concerned.
And because he's a kicker we're talking about a bonus in less than the $3M range on a contract in the 5/$10M+ range - the rest in salary which we know in the NFL is not worth the paper the contract is printed on when cut time comes. A $3M roster bonus prorated over 5 seasons is $600K per year on the cap. If he tanked and we have to cut him with 3 years remaining on his contract, his remaining dead money would be $1.8M (which could be spread over 2 seasons) - about what it will cost to cut Starks after one disasterous CB season alone.