Sure, but what if youve configured your waf or w/e incorrectly, what if someone gains access to another part of your network? Having sane firewall settings is always better than none at all.

A lot of people host websites created with a static site generator just fine without that extra layer. Unless your site has expensive to generate dynamic pages, lots of video content, or deep changelog pages (wiki history or forge commit diff's publicly available), it makes a lot of sense to keep it simple.

Putting everything behind Cloudflare is just cargo cult behaviour and learning all the ins and outs of it isnt any harder than learning a firewall (which is transferrable knowledge that can also protect your machines from each other).

If you're trying to be cost-effective, renting a VPS for a fixed term instead of using a IaaS platform that provides all these things just by clicking a checkbox, then caring about your firewall is a necessary part of building the system image that you deploy. You can of course set up your own private networking between systems and build the same type of architecture that a IaaS would provide, but to do that you're going to need to understand everything about linux networking and netfilter already.