the 300k doesn’t get “market dumped” it gets moved on to the vBNT curve and then burns the vBNT that it bought from that curve
it’s not future BNT that is locked by vBNT burning, it is the BNT that was just put on the vBNT curve that is locked
the v3 vBNT burning is a short term and long term benefit
that’s why it is better than the v2.1 vBNT burning which is only long term
you can read more about it in BIP15: Proposing Bancor 3