My guess is that keeping API access behind a paywall is a money maker for a few specific client sets; the ones that immediately come to mind are Trading Companies (both traditional and crypto,) marketing companies (analyze trends/etc,) as well as large orgs with some form of reputation to uphold (i.e. large enough where just one person manning the account is not feasible.)
Firehose access has been behind a paid wall for about a decade now (initially via Gnip, which they acquired). You'd have rate limits on Twitter Blue API access intended to support normal single-user activity.
