Last time I went to WDW I did a split stay at Universal. I considered the downside of not having the transportation to MCO included with my hotel, but ultimately the added cost of an Uber ride to the airport wasn't even remotely significant enough to make me reconsider.
I don't think the added expenses and/or hassle of losing ME is going to have a noticeable impact on attendance. It's a small blip when one looks at the cost of a WDW vacation. Disney fans are devoted. It's going to take more than this to send people offsite.
It's definitely a bummer and a degradation of the "Disney magic" that keeps people devoted. It's nice to step off the plane and feel like ones vacation has started. No luggage to deal with. No trying to find transportation. No stress of driving a rental car in a strange city. Just walk to the ME check in, easy and stress free.
Will this have some impact? I'm sure it will. Universal has some nice and affordable hotels, and it's no longer the simpler option to stick with WDW. With resorts charging for parking, the decision to use a rental car in the absence of ME is less appealing.
Ultimately, this offers no upside for guests, but most will not change their habits.