There have been exceptions, but in general, a country will provide its armed forces with the best arms the nation can afford to make or buy.
And usually when there are exceptions its a result of political, not military decisions. Or the one decision that is both, choosing only arms made in your country. Many nations have adopted, or kept arms that were inferior (in some ways) to other nations arms, because they were made there, and so the possible interruption of supply from a foreign nation was not an issue.
Even the US does this, to a degree. The few foreign designed arms we have adopted in the past century have all been required to be made in the US, or at least lip service paid to that account.
Nagants are "gone" forever, in the sense that all there are is all there are ever going to be. No one is making them, nor will anyone make them, as a commercial venture, again. Lots of guns in that class. Even the Luger, which is arguably more useful than the Nagant can't find a current, and enduring market for new guns. Everyone who has tried to re-introduce the Luger, including Mauser, has dropped it, after a brief production, as its simply not a money maker today.
SO I seriously doubt that between the obsolesence of the guns design, and its unusual ammo, it will make a comeback on the commercial market.
So go get one, If you want one, its an interesting piece of history. But don't think its more than an interesting piece of history, in practical terms, today.