These theme park designers did it to themselves - WDW in the '70s had lots of shade trees, benches and the coldest AC anywhere...many areas where people could cool off - and they actually removed the trees/benches - and got cheap with the AC, turning it higher - the opposite of what any intelligent designer would do in Florida summer heat - particularly with projected temp increases/climate change issues.
Personal opinion: it won't be long before most people will no longer be physically able to visit any of these Florida theme parks in summer over the next decade - the heat is beyond brutal.
I've been going since the 70s - and it's a lot different today.
There is a lot that could still be done to make the heat less of an issue, if they cared to invest. One very simple one for a start: I noticed on our trip a couple of weeks ago that almost none of the ride queue fans were running with the misters on. That might work in California, but in Florida the misters help compensate for the humidity; by putting water on the skin surface they aid greatly in cooling the body. (In high humidity sweating does not cool the body very much at all.)
WDW sells rain ponchos fairly cheaply, but price gouge on heat-protective items such as cooling towels, sunscreen, and sunhats. The prepared visitors buy cooling towels elsewhere and bring them along, but Disney charges 4X as much for them as one pays off-property. (I'm helping my DD plan a fundraiser to buy them to donate to farm workers, and we've been looking into buying them direct from the manufacturer. WITH a carabiner pouch on each one, an order of 1000 of them averages $.55 per towel, and the price goes down as the quantity goes up. Disney could sell them for $2 each and still make a solid profit.) Same goes for personal fan devices; they are all made in China and cost very little at wholesale, so if Disney wants to increase summer attendance, they should sell these devices at only a fraction over cost.
Lately (and I suspect the point is to sell more LL), I've witnessed queues being held outdoors, so that once you get inside the building the queue is nearly empty. That simply has to stop, because it's just flat dangerous, and will certainly deter repeat summer visits. ALL outdoor queue areas should be fully shaded during the day, with lots of artificial air circulation: no one should be standing in the sun while in a ride queue in summer. If they can do it for Haunted Mansion, they can do it for the rest of the rides.
The MK trees were cut back to increase fireworks viewing sightlines, but those sightlines are only needed at night, when shade is NOT needed. The obvious solution is to use more (LARGE) umbrellas and mechanized canopies in areas of the park that are within the planned sightlines. When the sun goes down you roll them back. (Added bonus if they use solar canopies.)
And speaking of the sun going down, the most obvious solution of all is to shift park hours, especially on weekends, to set the majority of open hours after 4pm. (Personally, I think that in summer it would be a great idea to rotate having one park open all night long one night each week, closing at perhaps 9 am, and only open regular hours the following evening.) Not only would this spare visitors the worst of the midday heat, but it would draw a lot more locals (and almost locals, such as Tampa and Gainesville folks), who work during the day and would rather visit the parks at night. It would also contrast strongly with USIOA, which always closes their parks comparatively early in order to drive visitors to CityWalk.
PS: Right now, it's very important that the company get a lot of revenue from the parks, because it's the only division that is presently really profitable. The Parks are propping up the rest of the Disney Company at the moment.
PPS: For what it's worth, I suspect that they could get enough volunteers to manage the overnight one park per week, especially if they threw in a bit of extra cash for taking that shift; after all, it gets the CMs out of the heat, too. There are also probably plenty of people who would be happy to take a p/t overnight gig once a week if their spouse worked days and was home with the kids overnight.