My personal experience is that in actual flying, WAAS has made little difference. I fly a Baron with a 530/430 both with WAAS and the Mooney above with the G1000 without WAAS. I put the WAAS in the Baron as soon as it became available for the 530/430. I fly about 300 hrs per year, mostly business trips. There is not one trip since I've had the WAAS in the Baron where I can say that the WAAS was a factor in my go/no go decision. I've flown several WAAS approaches to minimums but in every case there was an ILS alternate.
I'm pretty sure there'll be ILS approaches for at least another ten years. And at some point in this next ten years everyone who has a panel today, Garmin or otherwise, with or without WAAS, will likely upgrade it to a more modern panel with features, other than WAAS, that are not currently available.
So, my experience is, in real flying, WAAS is not an issue.
As far as being economically feasible to upgrade the '05 G1000 to WAAS, it costs about the same as installing a new 530/430 WAAS in any other airplane. If it's economically feasible to spend $40,000 on a panel upgrade in a $100,000 airplane then its certainly economically feasible to spend the same about in a $250,000 airplane. My point is that it's just unnecessary.