I suppose if you say it with a superior tone and act like you know better, sure. It's all in the delivery. If you ask it as a genuine question, assuming that you don't have the answer, then I can't see how it would be taken that way by most people.
No, it's not the delivery, it's the content and the fact that you're lying to save the other person's feelings. Specifically, the lie is that you think the solution is "reasonable" in the same breath that you raise an objection you believe puts it in serious doubt, if it's not outright fatal. You can deliver that in perfect sincerity and if I see what's going on, the condescension will come through crystal clear, because I'm not a child.
It doesn’t need to be a lie. You could be seeing that the proposed solution is way better than everything else out there including the existing solution. If only you could fix “<showstopper>” somehow.
We very often have discussions around which solution we take. Often we take the one that we don’t like as much but we know how to make it work. The other solution designs are kept so we can revisit them sometime (or as reference for when somebody says “why don’t you…?”) with fresh minds and then see a new way around the problems we saw before.
In my experience the “<showstopper>“ is almost never a law of physics but a shortcoming of a dependency, either it’s implementation or architecture, or just some assumptions that are or have become invalid later on.
