In my mind, £150 is a small price to pay for a fairly significant benefit of having an Amex Premium+ card.
I spend a significant chunk of money on my personal card each year - I use it wherever and whenever I can for every payment I may be making - but considering the BA Miles I get back, plus the 2for1 voucher (which is always put to seriously good use), I count it as money well spent.
Yes, I could switch to a card which doesn't cost me anything, but would I get the same level of benefits? Probably not. And are those benefits worth £150? Yes.
Would I prefer to get these benefits for £30 or less? Of course I would - but it's unlikely to happen.