Earlier today, Dan Counsell wrote:

The main reason Panic went with no upgrade pricing is because the Mac App Store still has no way of doing paid upgrades. If the Mac App Store didn’t exist then I’m 100% sure they would have offered upgrades. I believe Panic plan to launch Transmit 5 on the MAS at a later date, so not having to deal with upgrades gets rid of this one major headache.

It’s the rigidness of the Mac App Store that is still forcing Mac Indies to make these odd decisions when launching products. I wish Apple would introduce upgrade pricing on the MAS so developers can choose the revenue model that works for them and their customers rather than being dictated to.

I really doubt Panic will ever put Transmit into the Mac App Store. I think there's just too much going on with regard to sandboxing for it to work out. Remember when Panic pulled Coda 2.5 from the MAS? And I'm not sure they need upgrade pricing. They nicely explain in the FAQ for Transmit 5 that it's been 7 years since an update. If someone is going to get fussy over a lack of upgrade pricing, then maybe Panic doesn't even want those customers. In the end, if someone wants the best file transfer app on the Mac that money can buy, they'll end up buying Transmit anyway.

Dan is right in some sense... Upgrade pricing on the Mac App Store would be pretty great to have. If it were available, people would use it and benefit from it. But I'm not sure it's quite as necessary as he makes it out to be. No disrespect to Dan here, though. His company puts out some really amazing and beautiful Mac and iOS software, and I wish the best for him and his company.