Yes, I find this argument convincing… for those sigils which do allow composition at all. But there’s at least one which doesn’t, namely ISSUE! — we don’t see any ISSUE-BLOCK!s or anything like that. So there’s precedent for saying, ‘these sigils can’t be composed with other types because it makes no sense’.
From a broader perspective, I’ve been unhappy with the way some sigils combine with a specific set of types, while others only combine with one. I eventually rationalised it by saying that the sigils which can modify many types are those related to word-specific operations (getting/setting/binding), and the types they modify are either words or series of words.
Currently, there is only one exception to that trend: TYPE-*, which has no particular relationship to these operations, but can modify other datatypes nonetheless. And it shows, in that the usage of TYPE-GROUP! and TYPE-BLOCK! seems fairly ad-hoc, in that it’s hard to predict from the types themselves. And that situation makes me feel uncomfortable.
This is why I’m so unhappy with re-adding a whole series of ‘evaluate-to-self’ types: not only are they largely useless, they further muddy the waters by making the point of sigils even less clear. I can see only one reason to justify them, namely that they’re useful in dialects… but if ‘useful in dialects’ alone becomes sufficient reason to add new datatypes, then there’s a whole bunch more which we could add. And we’ve already established that adding those isn’t a great idea, largely because such types are only useful in such limited ways.