That’s good to know! As a non-programmer I don’t really know just how much is possible in theme components (a lot it seems, which is one of the many things I love about Discourse). So that’s pretty cool.
It’s true that in other software the [[ is retained and maintains some value even after the link is added. Or rather I should say, the [[ search doesn’t auto-fill a traditional link, but a specialized internal reference. Because multiple applications support that reference format, it’s portable in a variant of Markdown, so that’s pretty handy.
But anyway, in the case of Discourse [[ is just a familiar in-line shortcut, that happens to fortunately be unlikely to be triggered accidentally. I’d be happy with any other such text-based way to invoke in-line search that met similar criteria, but despite the differences in how it would work in Discourse vs. e.g. Roam, I do see some value in at least having the syntax be the same. Like I said, it’s something of a growing de facto standard. 
The other thing that occurs to me is that Discourse already has its own equivalent of internal links that get rendered in special ways: it’s how quoting works! So “post:10, topic:200454” will of course link to your reply to me here. Since this link function is intended to be specifically to internal topics, it could just use that and automatically display it as a link to the topic at render time. I can’t decide whether this is more in keeping with how Discourse does things, or less… 
On the one hand there is already this way of linking, this would just be a different way of invoking the link search-and-selection, and it is very similar to existing @ and # searches as I’ve mentioned. On the other hand it departs from existing linking behavior invoked by Ctrl+K, the toolbar, and other shortcuts. I think, though, that the “post:10” type of link is more similar to the [[ link concept concept as used in other apps, so I’d lean slightly that way… if I had any weight in the matter. 
I know of course that this is more theme component territory anyway, so perhaps I do! Maybe you can just weigh-in on whether “post:10” style linking could be done from a pop-up search in a theme component?