That is usually not feasible, since we usually provide support to 3rd parties (as a business).
Don’t get me wrong, I have your same idea: if you need GPOs is because you have Windows clients to manage; since your client environment is already Windows-based, an additional (client) license, which costs greatly less than a server one, shouldn’t be a burden anyways.
But this is true only if you manage a network with four or more PCs. Unfortunately we have customers which are really small (2/3 people working in an office) which are bound by GDPR to have personal accesses and thus need AD. To standardise the deployments on these customers (with the respect of the other bigger ones) you will deploy GPOs, so you need RSAT tools; some of the policies are replaceable with other workarounds (e.g. share mapping), some others are simply not economically justifiable, if done manually, for 2/3 PCs (like blocking USBs). And doing stuff manually on single PC is much more expensive (implementation, maintenance, complexity) than doing it once for all PCs.
So yes, I am sure 90% of the people, when faced with the higher cost of not having a Windows client machine for RSAT tools, will agree that a single license is better. But this is not always the case, and since we are discussing general support of GPO without proprietary products in the middle, I second that having an interface for managing GPOs would be better, even if the support is basic.