Not to argue, but Althorp was open to the public for many years before the current Earl inherited it, as a historic home. The upkeep on that size of an estate must be tremendous.
People want to visit Althorp to pay tribute to Diana. If the Earl had refused, that would have sparked another type of bad press. I'm no fan of his, but it seems that he is damned if he does, and damned if he doesn't.
I haven't been to Althorp, but have seen enough pictures of the exhibits there to know it is tastefully done, and extensive. If he is cashing in on his sister's popularity, then, at least, he is allowing people to pay tribute to her in an attractive setting, and he tries to give them some value for their money. It's not as if, in visiting Althorp, people are only given a short glimpse of the Island and a couple of tacky items on display.