Well, since this has turned into a decent discussion thread I'll throw out my qualifying idea. Keep in mind the numbers I've used are arbitrary and would probably need to be adjusted based on conditions, course played etc.
I was thinking something like a free monthly, bi-monthly or quarterly Q-School tournament (no prizes or credits, just a qualifier). Set an entry requirement of a 66 average and set the Pro Tier "pass score" at let's also say 66 for argument's sake. Have the tournament and let everyone shoot their 18 holes rounds. Then the top 50% (plus ties) of the group who shoots 66 or lower earns their "Tour Card" and advances into the Pro Tier for that time period. Once your "Tour Card" expires, you'd be required to re-qualify in another Q-School tournament down the road. If you cannot re-qualify, you are dropped back into the next lowest tier.
If you win any Pro Tier tournaments (whether it's 1 or 15 of 'em) then you receive an exemption from the very next qualifier. In essence, you're not required to re-qualify and your "Tour Card" lives on for another 3 months or whatever the time period is and you keep playing in the Pro Tier. This contingency would ensure that the folks that belong in the Pro Tier are certain to remain there.
In order for this to work, as others have said, Pro Tier prizes need to staggeringly more valuable than the other two tiers are right now. The incentive to advance must be present for everyone.
You could even take it a step further and introduce scoring averages for every course instead of one scoring average to rule all. This would force players to carry the required scoring average on the actual qualifying course in order to enter the Q-School tournament. It would prevent folks from sandbagging on the easier courses to lower the overall average and prevent them from back-dooring into the qualifier.
This is a very rough idea still and obviously needs work to tighten up the details, but I think that it would introduce a very realistic dynamic into the game. Players on the PGA Tour must earn their cards and the rights to play with the big boys, so should we.