Currently the minimum is a SEER of 13, but I've seen units as high as 22 advertised.
A SEER 13-16 system might last 15-20 years (if you're lucky), while a SEER 22 system could break down much earlier at about the 10 year mark. You can sometimes replace specific parts, but depending on what breaks, it may not be cost effective.
In short, the only way we're moving to more efficient AC systems is if costs come down significantly, we see some major innovation in the field, or the government provides significant financial incentives.
As an aside, I also think we could achieve energy efficiencies by increasing the insulation in houses. It's unbelievable how much hot/cool air is lost through windows, doors, crawl spaces, and attics.
Brushless / electronically commutated DC motors are pretty interesting, though: https://en.wikipedia.org/wiki/Brushless_DC_electric_motor
These motors are microcontroller-based. Apparently there was a rash of bad GE X13 motors in the early 2000's traced to bad thermistors. Unfortunately the board is potted in an insulating compound that makes board-level repair rather impractical.
My first motor's controller failed after about 8 years. It was cheaper to just buy a new motor vs. replacing only the controller. The second motor's rotor started grinding against the stator after about 13 months. I pulled the controller from that motor and installed it on the first motor (since it was near-perfect duplicate of the original motor). That ran for 14 months before the second controller failed.
I've been running the third (Nidec) motor for about 2 months. I'll be interested to see how to holds up long-term.
We went from SEER 10 to SEER 15. With the rebalancing that the contractor did, our house is much more comfortable with the thermostat at 80 than it was under the old system at 73.
With 2008 not being so long ago, neither job security nor resale value are that promising. We're still rebuilding consumer confidence after nine years. The economic recovery in the US is largely in a handful of major metropolitan areas pulling up the averages, with large swathes of the country still struggling. This needs to improve or some government incentive program like you mention must take it into account if we're going to sell capital-intensive home upgrades.
You can get ground source heat pumps that are 50 SEER, 5.3 COP (and they'll provide all your hot water, too)
There used to be a federal tax incentive of 30% of the cost, no max, but it expired at the end of 2016.
Everyone knows GS heat pumps are more efficient.
This seems to be primarily an issue with copper coils. I was bitten by this with a brand new system that I had installed about 9 or 10 years ago. It was pretty tough to eat the labor costs of having a new coil installed (that was covered under warranty) as well as a complete refrigerant fill (at the time ~$900 for the full R410A fill) on a system that was less than one year old. Its been good since then though!
EDIT: I should mention that SEER uses different units for each part of the ratio -- BTUs of heat moved vs watt-hours expended. COP is dimensionless, and EER is just SEER but for a specific operating point. SEER extends the measurement through an entire "season", and so it theoretically gives a more realistic picture of overall efficiency.
Anytime there are multiple standards for manufacturers to choose from, customer confusion is guaranteed.