Does the reason why the service is no longer accessible matter?
Yes.
Microsoft Silverlight will no longer be supported sometime in 2020, and without that the tools can't function.
In this case people have spent hundreds of dollars on purchasing books as well, and I would imagine for a fair amount of them they would also be their sole copies.
Thankfully, when DnDBeyond does vanish into the aether of the web—assuming no one has turned a webscraper onto it and copied it's files to make an offline version—it will be many years into 6e.
At that point, anyone who only had digital copies of 5e but still wants to play should be able to find cheap, used copies of the 5e books they lack physical copies of. Probably for cheaper than you can get them now...
Plus, on the DMsGuild, WotC and One Book Shelf have slowly been adding Print on Demand copies of books from older editions. Half a decade into 6e, you can expect to be able to get physical copies of 5e books printed to order at will.
Again, assuming they're still playing 5e.
Them eventually becoming unavailable is a very valid concern. One that I think is just flippant to dismiss with "Nobody plays old games anyway, lol".
Thankfully, the D&D Beyond website is just a website and doesn't make use of Flash or Silverlight, and is relatively future proof.
Given WotC kept the digital tools for their least popular edition running for eight years after that edition ended until the technology that drove it went away, I can imagine them allowing DnDBeyond to stay up but just ending new subscriptions/ purchases for a similar length of time. At the very least.
So those digital purchases might be safe until the better part of a decade into 6th Edition. Which at the current rate of things, might well be 2030 or 2035.
So someone who buys the all-in Legendary Bundle right now will pay $663. By the time the service theoretically ends in 2035—if ever—they will have had the products for 180 months or $3 per month. It would have cost them a cup of Starbucks each month, and for that price they got fifteen years of access to D&D books. That's pretty good. They got their money's worth.
It doesn't seem entirely unfair that it eventually goes away. Nothing lasts forever.