They're just legacy now IMO and their long term support requirements are a result of this, companies that haven't gotten rid of them by now aren't likely to do it any time soon.
I hate seeing them go. I wasn't such a fan of Solaris but I was of HP-UX. But its days are over. It doesn't even run on x86 or x64 and HP has been paying Intel huge money to keep itanium on life support, which is running out now if it hasn't already.
At least Solaris had an Intel port but it too is very rare now.