We look at trips with and without the kids as being in totally different tiers (both in terms of what we can do and how much money we're willing to spend), but no destination is off limits for either option. So we have no problem doing Disney without the kids (and we have done this in the past and are planning to again), just like we have no problem taking the kids to Europe rather than "saving" it as a couples trip.
We look at Disney trips with the kids as being about the kids. We focus on what they want to do, move at their speed, etc. If it's an extended family trip where grandparents are coming with, we have enough wiggle room to trade kids between the adults so everyone gets the chance to have some downtime. But an adult-only trip is a chance to focus solely on what you want to do, experience nicer dining, stay at a nicer resort, etc. They're totally different types of trips, and IMO, they're both worthwhile vacations.
Some thoughts on your daughter's age: Yes, it's true that she won't remember a trip at this age. BUT she will have a great time, and YOU will always remember seeing her have that magical experience. I took my 2- and 4-year-old daughters with my mom for their first trip, and I wouldn't do it differently even knowing how little they'll remember. My own first trip was at age 5 that I have no recollection of, and my mom says the same: no regrets taking me so young.
Obviously I'm an advocate for both taking kids of any age to Disney and having adult-only trips.
I think it might help your wife to think of it as less of an all-or-nothing thing and more of a vacation spot that everyone can enjoy at different times. Just because everyone
would have a fun time doesn't mean everyone
needs to go on every trip!
Like others have said, it also depends on if you have someone you trust and know well to watch your daughter while you're gone. We've taken a kid-free vacation every year since our oldest was born, because we really need that time away from our kids. So going to Disney without them doesn't feel like a big deal because we know we're going
somewhere without them, and Disney is just another vacation spot. But some people hate to travel without their kids regardless of the destination. It's all such a personal thing.