Look, if you want a "real" WAF capability you buy something like Imperva and manage the care and feeding of a team of say 2-4 ppl who understand web app vulnerabilities in depth, AND know the tool. Your SOC/NOC will need training and procedures too. Fully loaded an average enterprise will pay > $1M a year to maintain the capability if you look at the TCO carefully.
There are environments where this makes sense. Banks are the classic - they have the money, they care about the risk, lots of places have systems still on java 8 and take > 1 month to deploy code fixes.
The biggest practical problem in my experience is that it's almost impossible to keep good people maintaining an enterprise WAF. Anyone with a deep enough understanding of web app security and network infra to do the job properly will get bored in the role and leave. WAFs are usually barely maintained for this reason.
Another major issue is alert handling - there's nobody with enough context. The SOC staff usually understand nothing (not the vuln, not web, not the app). The devs don't understand the vuln and the security team don't understand the app so tickets go round in circles with nobody able to understand if an alert is an FP or an actual issue. Eventually alerts start getting quietly dropped on the floor.