I think their main goal is to reduce their platform support rather than diversify it. Using a single desktop app that’s basically webmail inside a wrapper allows them to support basically any OS with a single app.

The downside for users is that reducing support for IMAP risks of causing lock-in. For better or worse IMAP is the standard way of accessing your messages for backup or to migrate to another provider and discontinuing it would be a very bad move.

…That doesn’t mean they won’t do it though. For example they have an export feature buried somewhere on their website. They could decide to kill the bridge & IMAP and only offer the export as the sole means of egress. In theory you would not be locked in, in practice who knows what the limitations might be. From a technical point of view, accessing your messages “live” through an IMAP bridge is a lot more secure than requesting a zip of all your messages (which needs to be stored temporarily who knows where).