I’ve been to both of these places. Birkenau is pretty much a ruin in the middle of the country anyway. There is not much to preserve left except the entrance building where people got off the train and were taken off to their doom for the most part.
Auschwitz is much smaller and there have been preservation efforts going on there for years because while it was in better shape than Birkenau and some others (because it had been built years before as an Army barracks and was therefore not a hasty construction), it has been decaying. It is a HUGE tourist draw. They get thousands of people going through there, most of whom bus in from Krakow. I hate to sound mercenary, but in addition to the historic aspect, Poland makes a lot of money from the tourists. Not so much at the camp itself (though there are some restaurants and souvenir stands, which is a little creepy as they sell toys and sweaters and stuff, not just books and postcards of the camp) but from people staying in Krakow, spending money there, and hiring a tour guide or tour bus to take them out to see Auschwitz. I predict if they decided to get rid of most of the camps, they’d still want to keep at least the small Auschwitz I with the famous Arbeit Macht Frei sign.