There was huge skepticism in the industry that landing and re-using first stage boosters was viable. Tony Bruno of ULA in 1015, 7 months before SpaceX recoverd their first booster, on their plan to recover the engine module of Vulcan:

"Our concept for reusability is inside that mathematics and realizes that maybe reusability does not start with entire stages,"

It's not that they thought it was impossible to do at all, but they thought the weight requirement of fuel, landing legs, heat protection, etc would be so high that it would eat up too much of your payload capacity.

Scepticism is way too often confused with negativity, pessimism or even outright hostility.

It's also often overlooked that a solution might not be the best fit for everyone. Just like it's best for the Falcon 9 to discard the second stage.

Right, the reason ULA can't adapt Vulcan to land the first stage propulsively is it can't throttle down low enough. It's only barely possible with the F9, and that's because it uses a cluster of smaller engines and happens to have one in the middle.

That wasn't a design choice driven by reusability, originally SpaceX were aiming to recover the stage using parachutes. The original plan was a falcon with a cluster of 5 engines in a ring. Using a cluster of smaller engines was for two reasons. First because they didn't have the resources to develop a new larger engine. Second they wanted to mass produce the engines to get production efficiencies and a bigger run of smaller engines worked better for that.

It just worked out serendipitously that this lead to a design that was very well suited to adapting for propulsive landings. If they'd developed F5 first instead, they wouldn't have been able to land it because without a single centre engine they would need to use at least 2 engines to land, and the thrust would be way too high. Even with a design with a centre engine, the Merlin would be too powerful for the weight of an F5.

This isn't a criticism, it's just the way things worked out.