No your not missing anything. If you value avios at 1p/point you need to save 30k avios to break even with the £300 fee for the voucher.
I will save about 80k avios using mine, so it's good value for me, but only if I don't have to spend £5k extra that I don't need to in order to get the voucher. My normal monthly card spend on living expenses without exceptional items (like paying for holidays in J) is £800/month so I am one of those in the middle who will be impacted by this change.