Congratulations! I almost purchased one on Monday, might return to the dealer today to finalize the deal. I'm struggling with the decision as I currently own an Outback and the Volt would be replacing it. The Outback is so practical (and fun, it's turbo charged) I'm finding it difficult to replace it. Since I don't drive much the gas savings with the Volt will never approach the acquisition cost. So I need to decide based on other factors (such as the Outback having 155K miles and being 12 years old)
You didn't ask my opinion, but here it is anyway, haha.
If you are committed to remaining a 2 (or more) car household, I cannot see any reason to postpone buying a Volt where you will get $12.5K off a $30,000 car in Colorado. That's like freaking Kia Rio money. If you're considering downsizing to one car after the Subie dies, then by all means, drive it to death.
This is particularly true when 1) Your state has been somewhat ambivalent about the future of the extremely expensive state program, and 2) $7500 of the rebate/tax credit money likely goes away by the end of 2018, regardless of what Colorado does. Chevy will certainly have to respond to this with more aggressive incentives of some kind, but I doubt they'll be offering $7500 off MSRP.
Frankly, I think the smartest thing you can do is either buy the Volt and keep your Subaru as a third car (or your kid's car for the future if that's on the horizon) or use it as a "final bargaining chip" at the negotiating table. Reach your deal on the new car price, say you want 4-5K on your trade, talk about how reliable it's been, hem and haw about whether you really need a new car or not. If they give it to you, great, you just sweetened your deal even more. If not, get the Volt, move on, bring your Subaru home and drive it into the ground on errands where you need more space.
The reality is, at 155K miles, you're likely going to need a new car sooner rather than later. Nobody's forcing you to trade in the Subaru though. Take it out of the decision-making tree and decide if you want/like a Volt (or other PHEV/BEV). Then decide if you want $12,500 off your car automatically and all of the chips in your pile at the negotiating table, or you want to pay something much closer to list price in 2018.
Your points for the dealer are:
1) I have a car I like. I don't need a new one, but I am open to making a deal.
2) I'd like to keep my reliable wagon for snowy days/kids/hardware store runs.
3) But if you can sweeten the deal on my trade, I'd like to have it all done with today, not inconvenience myself with coming back to pick it up, etc.
One last point: If you wait for the Subaru to die, you HAVE to replace it, which puts you in a much weaker negotiating position than merely being interested in a car at the right price.