Think about how little it costs to be a free hosting provider for a handful of 15MB downloads, maybe one per major version, and just for people that paid you in the past. It's next to nothing. In particular, cloudflare will give you 10GB of free object storage with unlimited bandwidth and 10 million downloads a month. So yes, they should host the installer forever.

To put it another way, the cost to provide downloads forever instead of just once is less than a penny. It should be built in to the purchase.