Just to be clear - I do understand your points.
That said - it is no different anywhere else. If as site owns the file (like other marketplaces) or if they simply host other peoples' work (like envato), if a file is then removed, the availability to download the file and any future compatibility updates are no longer accessible. My point is that it is the same on just about all marketplaces like this.
What guarantee do you get buying the theme directly from the author? What if they just decide to stop supporting or updating a file? Ignore your emails?
I may be misunderstanding you but I don't get how this would be any different?
It's worth noting that the author didn't just move to the other marketplace - they have been selling the item there since 2015. Also they are an exclusive author so they can't be selling it in the two places at the same time.
People buying $50 themes (worth thousands) rather than paying an agency etc. must accept that the level of service and guarantees cannot be the same. Especially as having 6 months support and lifetime updates while the file is live for FREE is not a bad deal at all.
Again I do understand where you are coming from but It's always been this way and I highly doubt it would change because either route of owning the themes or enforcing authors to support items forever would destroy the marketplace and drive both authors and buyers away.