It's also completely reasonable to say "We shouldn't do Y". The response to everything shouldn't be to go immediately into solution space and list off trade-off

It's also completely reasonable to say "We shouldn't do Y". The response to everything shouldn't be to go immediately into solution space and list off trade-offs on the assumption that Y is happening. With people you trust for both their integrity and their expertise, "we shouldn't do Y" is a reasonable shorthand for "there are a pile of valid reasons we shouldn't do Y, and you and I both know I can list them for you off the top of my head, and it might well be possible anyway but it would take a lot more work and trade-offs than you might expect". That isn't the end of the conversation; if the response to "we shouldn't do Y" is "it'll be a pain if we can't do Y", then you can explore solution space together, where that solution space includes both trade-offs to support doing Y and alternatives to doing Y. Use experts for their expertise, rather than the equivalent of "I don't care, make it happen". Or, in short: yes, of course the DBA doesn't unilaterally dictate the architecture, but maybe listen to their expertise anyway.