OP - we basically did exactly what you are suggesting, and overall we've been very happy. Our InstantPot gets used 3-4x week and has held up incredibly well. My only qualm is that, being round, it doesn't fit roasts as easily as our oval slowcooker. FWIW we have the 6q 'Duo' model. Works very well as a rice cooker, slow cooker and pressure cooker, with the added benefit that we can sauté things in-pot before switching over to pressure/slow cooker settings. Finding out that we could skip the soak with dried beans was a revelation and has freed up a lot of pantry space from using canned beans. The timer is also much improved from our slow-cooker; we can set just how many minutes/hours we want it to cook before it either shuts off or holds temp. It also uses far less energy to cook the same amount of food
About 1-2x per week it gets used as an overnight yogurt maker too, which has been a big and unexpected bonus.
In short we like it so much we're considering getting a second smaller one on holiday sale so that we can do two things simultaneously.
Re: Longevity - so far so good. The InstantPot brand seems pretty robust, in about 2 years its cooked a few hundred dishes and we've had zero issues. We replaced the silicone gasket a couple months ago, but that's because our cat used it as a chew-toy when it was on the drain-board. I'm guessing the electronic control panel will fail first, but hopefully not for at least 5+ years (it seems pretty similar to what a microwave or modern oven use, so don't know why it would fail sooner). Time will tell I suppose.