Here is a proposal. It has this HUGE upside and this SMALL downside
response: because of this HUGE downside, and the TINY upside, I reject this proposal
If they said "what if the upside wasn't as big as you stated but the downside is larger than you stated" at least you could discuss the evidence. But people love to leap on problems and devils-advocate them into the ground.
You see this all the time in IETF mailing lists. I'm not talking about nit-picking during working group last call on a standard, thats justified. People who simply want to be contrarian, take a devils-advocate stance, leap on any stated downside and on the premise its the proof, destroy the original idea, irrespective of the relative merits pro and con.
So, "we should move to Postgres because of its support of IPv6 and JSON" dies on "but the sheer amount of code we have in MySQL makes this untenable" -which is not a good argument, given the budget and willingness to incur the cost. It doesn't address the upsides of the move at all. Or "but we don't know all the places which use the old SQL forms" which is true, but presupposes we couldn't handle case-by-case the legacy calls into the old SQL binding, or find some way to uncover them.
The negative case arguments used, typically are shorthand for "I don't want to think about this"