Hey, if I can use a 6-7 year old machine every single day (and do everything I need), then I don't want to hear any shit about a
three-year-old MacBook Pro somehow not being up for the task.
Go through with your upgrades, drew. You have your reasons, and you know your plans/budget better than any of ever could.
The way I see it, USB 3.0 and Retina aren't, in and of themselves, two "deal sealers" for me.
When you do finally make a proper upgrade in a few years, guess what? They'll be there waiting for you (possibly even USB 4.0?, and the Retina Displays will most likely be even better and more affordable). You're not going to miss that train. You're merely holding off on catching it for another few years.
You can't, or don't want to, spend the money now for a full-on replacement? That's totally understandable. These MacBook Pros are the most easily upgradable of all Macs (minus the towers). You can't do the RAM in the smaller iMacs anymore, you can't do the hard drive in either (and haven't been able to in quite some time). The mini only allows easy RAM access and the Air is, for the most part, welded shut even tighter than the current iMacs.
So if you have something like a 2011 MacBook where the RAM and hard drive are incredibly easy to swap out (and the manuals even walk you through it and show you how, so you know it's not "major surgery"), and that $400-500 is within your means and give you the boost you're looking for, then go for it!
As Dorian says, be sure to look around...that RAM sounds very high (I haven't been keeping up; did RAM shoot back up recently?). But other than that, you're doing what I'd do.
Hey, if I could easily throw a 256GB SSD into this iMac, I'd do it tomorrow and keep using the damn thing for another 2-3 years! I'd enjoy a decent boost for $200-300 vs. laying out $1,500+ for something I may not even need at this point (or take full advantage of).