Is there any point in having a separate modem and router?
What are you trying to accomplish? Better range, more router tools? Often people use their own router versus the supplied router/modem of their internet service provider for enhanced wifi range. I use the cable company’s modem/router, but had the router bridged so I can use my own router system.
I have done it this way for years. Just upgraded to a mesh wifi router modular system called eero. Previously was using an AC1900 ASUS. The eero is more effective in covering my entire house with usable signal. The separates are more expensive, but it comes down to how well you can get the supplied device to work, versus something more custom. There are other tools for increased range available, as well as hard wiring, wifi bridges, etc. Anything extra adds to the cost. Hope that helps.
There are some effective tools to deliver ethernet hookups without retrofitting with Cat cabling. MoCA and Powerline adapters can be effective tools for connecting ethernet cabling across the house. MoCA is ethernet over coaxial cable, and Powerline is ethernet over powerline.
If I had a penny for every issue I’ve traced back to a router provided by the cable company or ISP…
I’m sure some ISPs provide decent hardware, and I’m sure my view is skewed by years of looking at troublesome networks supporting Sooloos and Roon – when everything is working great, we don’t hear about it 
That said, I think a decent 802.11ac wifi router is a really good investment, particularly if you are using multiple audio outputs or control points. This is doubly true if any of them run on WiFi. Your ISPs job is to get internet into your home, and trusting them to manage your local network can be a frustrating and time-consuming roll of the dice, especially if network troubleshooting is needed down the line.
A little off topic, but Fing is coming out with a wiz-bang app to help tune your network system. The useful Fing app is free, but the new tools (upgrade) being added will have a fee attached.
I keep my modem and router separate. My Netgear Nighthawk router has been a great investment… super reliable with a great feature set. The last thing I want is any reason to have to call the brain trust at Comcast Support if I have a home networking issue… e.g. power cycle, power cycle, ok we’ll send a tech out next week to make things worse.
edit: I also own my own EMTA, at $8/month (or whatever Comcast charges now) it didn’t take long for it to pay for itself.
ON Demand and other TV options are lost if you put the ISP’s router into bridge mode to your own equipment. At least for some of us.
The technologies change at different rates – the modems more slowly, the routers more quickly. I started with my own router and upgraded it to a current one recently. I started with Comcast’s modem but bought one on eBay to replace it. I save $8 a month in rental fees, so it paid for itself in a year. That was two years ago. Not a pot of gold, but saved money nonetheless.