I'd put it this way: It could be made to work, but IMO all current RDBMS have the same problems with terrible worst case guarantees. Query planners have not enough degrees of freedom - maybe with a new concept like automated indices it could be made to work with acceptable average and worst case guarantees.
But I do agree that PostgreSQL has way too little tools to nail down the performance even though they have downsides. Tools like pinning execution plans would be nice, as it has less severe worst case behaviors. As would be the ability to just pass the execution plan directly, although that would have severe cross-version compatibility implications and security will also be hard to nail down after the fact because of all the "can't happen" assumptions sprinkled around in executor code. And even just plain plan hints would be great to have, be it the heavy handed "join in this order", "use this index", or the more graceful "this clause is way less selective than you think", "this clause is functionally dependent on that one" or "assume there is correlation between ordering and predicates".