Actually, it's been mentioned a hundred times already. UMD's have been deprecated, Sony was planning to move away from them for ages now - since the PSP Go.
The Vita is perfectly backwards compatible, to the point that Japan had a UMD Transfer program. Not sure why they didn't run the program in other regions, but that's irrelevant, it is backwards compatible. Optical drives are bulky and suck the battery dry, there's no need to use them whatsoever in this day and age. Just use the PSN store PSP downloads, it's simple as that.
People buy new consoles to buy new games - backwards compatibility is a nice addition, but an addition none the less. Does the 3DS have a GBA slot? Did the DS have a GBC slot? Did the SNES have a NES slot? Did the N64 have a SNES slot? No on all accounts? So why do we pick on the poor Vita for not having a deprecated storage slot?
We might as well start whining that nothing supports CF cards nowadays.