No-date Subs are much more appealing, I agree.
Did you really expect that a dealer would just happily make a trade at what you consider market value? They are in the business to make a profit, and in a trade, they want to make a profit on both ends. It's a job for them, they want to make money. The price when new is irrelevant, the only thing that matters is what you can negotiate.
If you think it's worth more, sell it yourself, and then buy the no-date Sub. If you want to avoid the hassle of selling and buying, then take the loss.
In your place, I'd probably buy a nice no-date Sub while prices are low, live with both watches for a while, and then sell the one I liked less, preferably in a couple of years after prices have recovered.