A miner including the transactions but withholding the data would 1) collect the fees, 2) see the block propagate fine among unupgraded nodes, 3) use less bandwidth. Other nodes might propagate the data for their transaction seeing the block, but the block itself would not propagate with SegData, since nobody has the entire SegData. This would probably lead to reduced visibility of the SegData and reduced data availability. So, data embedders would be be incentivized to use witness data instead of SegData for the same cost which guarantees data availability.
I just don’t see how this proposal solves any problem for any party.