To get back to the main issue - the reason for a licence is first to denote a certain minimum level of knowledge or competence and second to give the issuing authority something to take away if that minimum knowledge or competence is not maintained.
Given that ATSAs don't just walk in off the street but are employed by a small number of easily identifiable companies surely it's possible to achieve all of the safety measures within each individual ATC unit. It's been possible for years - although it's been done better by some units than others. OJTI responsibilities and all other special tasks can be done locally.
Let's get real. There's no need for a licence for ATSA - there are lots of other ways to ensure they are competent. In spite of claims to the contrary, the only reason anyone would want an ATSA licence is either for some form of status or to justify more money.
The same is true of controllers too. The only reason that they have licenses is because it's an international regulation. It would be interesting to guess whether if the regulation wasn't there, would controllers have licenses or just bits of paper authorising them to do the job - there really isn't a lot of difference if all you want to do is the job.
And btw, like it or not, even though controllers may not be able to do ATSA jobs as quickly, the job is supporting the controller and so they could do it if they needed to - or at least they certainly should be able to!