Just checking... did they actually make Mythic Dungeons easy enough to be done by the bulk of the LFR crowd? (I haven't been in any.)
And don't you need a pre-made group for Mythic Dungeons? If so, people not doing pre-mades for raiding generally won't be doing it for dungeons... so it would not be an effective substitute (or progression path).
The Valor thing is kind of a non-issue though. Since it is entirely possible to ration out the Valor to match the same rate of gear acquisition as random drops, it really would not change anything other than maybe having a psychological impact. (If I were doing LFR, I would prefer a steady pace, but a lot of people like the feel of random better.) As long as the things time out the same, it does not matter much though.