A device is only functional so long as there is a network to support it, and 3G is definitely on its way out. If 3G is where your device tops out, network technology-wise, its days are numbered.
I was a flip phone user way past the time when they were "the thing" (and I mean up until early 2018), but I would never consider getting a flip phone now. They will often cost more than low-end smartphones, and you need not use data service on a smartphone.
I just bought a ZTE Vantage (and with the intention of using it as a music player, never activating it) for under $30 US. It's hard to find a flip-phone for that these days, as they've become a niche item.