I recommend separate cable-modem and router components.
If either goes bad, or you switch away from cable, you only need to replace one component.
I just upgraded both & did quite a bit of research before selecting them. Here’s what I bought & recommend:
TP-LINK TC-7620 DOCSIS 3.0, 16x4 Cable Modem - Newegg $70 new
ASUS RT-N66W N900 Router - Newegg $50 after rebate, new
BTW my 2nd choice modem was the ARRIS SB6183 Cable Modem.
I didn’t go with it because Motorola no longer makes it, and the reliability is no longer what it was.
The TC-7620 is also a newer design than the SB6183. (I’m using Comcast)
My TC-7620 & RT-N66W both run cool.
For the router, stick with Asus. They’re the most reliable & upgrade their firmware often. (This is important with all the hacking going on.) 2nd choice for the router would be the ASUS RT-AC68P. I wouldn’t buy a more expensive one than the RT-AC68P. If you have dead spots, add a router access point (AP).
I agree with using a separate cable-modem and router, they serve different functions. And, as you say, they have different maintenance and upgrade cycles.