To OP: what benefits were you expecting/hoping as DM?

I'm sure there is some devaluation of DM since the introduction of Amex DM's but from my time as DM, it's still quite different from GO (i.e. first versus biz lounge access, op-up priority, overall treatment/recognition on board, mileage redemption).

As to op-ups, the biggest factor is which routes you fly and having flexibility as to which flight you take if there are multiple flights to that destination on same day. Your op-up rate is probably due to lower yields in Y over the past year as I don't think they prioritise DM's based on revenues in terms of op-ups (there are suggestions that lower BN's have better chance but I don't think the system can op-up based on a DM's recent/past revenue).