I don't think custom backends are going away either, I do think it will become way less prevalent then it is today though, similarly how all programs used to do manual memory management, this hasn't gone away but most programmers now rely on garbage collection.

So much of backend work is redundant, login, user info management, allowing the creation of groups of users, user following, user data uploading or sharing. Not to mention all the DevOps, managing CloudFormation scripts, VPCs, proxies, load balancing, dns, ssl, cdn, database schema, the list goes on, and that's before there has been any original development at all.

With react native and electron (I haven't found the atom browser or vs code weirdly restricted) the web stack is moving into desktop territory and with serverless frameworks becoming more prevelent hopefully this will make writing original software easier with less redundant work.

As to your point about data processing needing a custom backend, I see the exact opposite. Generally all this data is just funneled into a generic hadoop cluster and can be manipulated any which way from there using spark or any number of data analysis tools.