Living people aren't some kind of exhibits in a museum. If they don't want to retain their existing language as living, they shouldn't be forced to - just as they shouldn't be forced to drop it.
Same thing with language purity. Loanwords etc are a natural part of language evolution. It's up to the community speaking the language to incorporate more loanwords - or not - as they speak it.
That's too simplistic. It's very hard to keep minority languages alive, even if you really desperately want to, when you're immersed in a majority language.
The Irish language is in danger even in the Gaeltacht areas where native Irish speakers live, because English is everywhere. Tech is almost exclusively English, most jobs are English-only, most media is in English (not to mention ads on YouTube), most news and politics happens in English (the primary sources at least), all the packaging on everything you buy is in English, it's much easier to find a solicitor who speaks English-legalese than Irish-dlítheanga, your kids pick up English from TV and life.
Before you know it, you and many of your acquaintances are a bit rusty, and conversations happen in English more and more. The language declines, and it's not what you wanted.
Well, yes, it's a community endeavor. But if the majority of the community want to keep it, they will have no problem doing so - most jobs and media would be in that language. OTOH if you find that everywhere you go, you have to use English because other people do it - well, those people are part of the community, too. If most Irish want to speak English in most contexts, why should they be forced not to for the benefit of the minority that wants to preserve the language? If the minority really feels so strongly about it, they can organize as a separate insular community, so that their children don't have to interact with other languages etc.
This equation is different when the dominant language is spoken by the minority, but that minority can overpower the majority for historical reasons (e.g. historical concentration of economic or political power in their hands) - a good example of that would be Quebec. But the proper solution to that is to prevent or fix the concentration of power, not to force anyone to speak or not speak what they want.
Same thing with language purity. Loanwords etc are a natural part of language evolution. It's up to the community speaking the language to incorporate more loanwords - or not - as they speak it.