I'm curious if you've ever worked as IT for a company, what % of companies uses either just AppLocker, or the bigger "component" DeviceGuard that includes AppLocker? How popular are these technologies?
90+ % of all companies within regulated industries use AppLocker (or third party equivalent products). Fewer use DeviceGuard within the smaller business sphere.
Most small companies not in regulated industries use much of any security, so the point about them using or not using default deny is moot.
AppLocker is 2nd Generation Microsoft Software Restriction Policy (SRP). SRP is ancient, but to this very day remains an extremely effective security layer. There is a large sysadmin/cybersec pro userbase that knows how to work with it. So it remains operationally relevant to hundreds of millions of companies and organizations.
Most any enterprise or government that is regulated uses AppLocker or DeviceGuard because the regulations mandate (require by regulation and/or law) the implementation of 1) blacklisting, whitelisting, or a hybrid (grey listing) - blocking via one of or a combination of the SRP variants, 2) attack surface reduction (ASR) which is partly achieved by disabling insecure and/or targeted-for-abuse functionality via one or a combination of the SRP variants, and 3) least functionality, which is again partly achieved by disabling unneeded or not-often-used functionality via one or a combination of the SRP variants.
AppLocker and DeviceGuard require Volume (E3 or E5) licenses. They are included in the base license subscriptions and are easy to manage.
A lot of security can be gained with a little judicious, shrewd, and disciplined use of AppLocker/SRP.
A huge amount of security is gained, with little impact to necessary productivity by stopping "users that want to use stuff" from using what they want to use or do.
Companies and governments do not like change. Particularly in IT because change is organizationally difficult and expensive. They will use "legacy" or "obsolete" versions of native Windows security forever.
If it works, it works. If it ain't broken, it doesn't need to be fixed. Less is way, way, way more. Always make knowledgeable, experienced sysadmin make the decisions; never allow a user to do a damn thing without cybersec approval and never - ever - allow a user to make a decision even if you provide them all the alerts, notifications, and infos in the world. Users are always the problem. ALWAYS.