Matt:
Sam Snead finished T3rd at the 1974 PGA -- behind two guys named Trevino and Nicklaus, and a shot ahead of someone named Player (all of them in the absolute prime of their careers) -- at the age of 62. I'm pretty sure he got in the tournament because the year before, at age 61, he finished T9 (at wonderful Canterbury, where I had the privilege of watching him for part of a round). He got in THAT tournament, me thinks, because in the 1972 PGA, he finished T4 (alongside Casper and Floyd, and only three shots shy of winner Player) at the age of 60.
OK, maybe the Slammer was an anomaly. But I think you're argument would be strengthened if based solely on competitiveness, not age. You say the "essence" of the championship is featuring players capable of winning it. That may be, but that's not the totality of the event. I think the R&A makes a pretty big deal of honoring the highest finishing amateur in the tournament, don't they? I know of no amateur in this year's field remotely capable of winning the Open. But they ought to have an honored spot in the field (particularly at Hoylake, of all UK venues), as should past champions. The also provide some spots for local (UK-based) qualifiers, few if any capable of winning. But don't local fans -- and by extension, their local golfers -- deserve at least a few spots for homegrown talent to shine in such a premier event.
In his prime, was their a better ambassador for the game than Seve? He practically, single-handedly, rescued the Ryder Cup from oblivion. That the R&A gives him a spot doesn't strike me as a breach worth reforming. (Besides, it's daylight forever this time of year in the UK. What's a few more threesomes? And I bet Ivor Robson loved calling off his name on the tee today.)