To address the argument from authority, the presumption there is that the existing expertise has been self selected by a rational and efficient market, which in the government contracting sphere is very much not necessarily rational, and I think you'd have a hard time finding an efficient market anywhere.

It's pretty clear that from first principles, reusable rockets are a win[0]. The question of actualizing those principles can only be assessed by actually doing it, and is likely to be highly empirical. Unless there have been secret launches we don't know about, the ESA cannot possibly have good insight about whether or not it's valuable.

[0] then why haven't they done it yet? As a constrained market, none of the contractors who normally build rockets have had to yet. Moreover, I presume a lot of the computational engineering would not have been so easy 10-20 years ago, much less in a large contracting company with established folkways.