The main difference between a forum and a link aggregator isn't how comments are ordered. In fact, a link aggregator doesn't even need comments; it needs just the ability to share links.
So Lemmy already fills both purposes, with or without changes on the sorting system.
That said, your idea could be split into two features:
- sorting order, oldest first. This is already implemented. (It's damn useful to watch how a discussion evolved.)
- "flat" (non-nested) view for comments. Could you exemplify a benefit of this feature?