Call Microsoft about a bug and tell me how well their support works for you.

Pretty well, actually.

Every Windows version was shit when it came out, then became good through updates by the time the next version came out.
Except for Windows ME and 8, which were just shit.

You don’t know what your ISP-provided router does exactly. It may let some traffic through from the outside. It may get an over-the-air firmware update or config change at any time from your ISP. It definitely has well-known, unfixed vulnerabilities.

Also, if you rely on NAT, you have to have 100% trust in all devices that are inside your network.

Thank you for your valuable contribution.