This all seems to have got a bit wild since my OP (I'm sure I've just seen something suggesting Onana is no worse than Schmeichel!) so, expanding a bit, here's where I was coming from...
We had a goalkeeping problem in the guise of DDG, because he needed replacing and, where a well-run club would have had a plan to do that and executed that plan, we delayed before ETH - you've got to assume he was the big influence - decided Onana was the solution. Except he wasn't and the keeper problem remains.
I was definitely amongst those who thought we could get by for a bit. That's a 'get by' thinking process where I, and by the look of it, others, saw we had bigger priorities, a constrained budget and could realistically be able to get to the stage of at least competing for the top 4, with Onana in goal. Why have I changed my mind? Because the evidence is clear that he's not good enough. Yes, he can make some exceptional saves (exception seems the key word here), yes, he's comfortable with the ball at his feet but he's just unreliable. He's a limited shot-stopper, seems to have a flawed technique, fails to command his area and has an above average tendency to produce howlers. You can't have that.
Funnily enough, some of the debate around Schmeichel and VDS has reinforced my thinking.
Schmeichel was a transformational player for us. Sure, he conceded a few poor goals, particularly early on - and they might now stand-out more because of his reputation, as there's a tendency to want to prove that even a world class talent can make 'mistakes' - but he was undoubtedly one of the best keepers the PL has seen and, for me, one of the best ever. He had huge physical presence, amazing shot-topping, dominated his area, and often dominated opposing strikers. I remember watching so many games where a forward got a chance against us and you were quite relaxed about it because you knew he'd make the save. Contrast that with watching a set-piece this season or last, where you're completely lacking in confidence over what Onana will (or won't) do.
I didn't see as much of VDS as we lived outside of the UK during much of that time, however, he was also a transformational keeper, who was a big part of our success at the end of the SAF era - just think about the difference from the chaos of Bosnich, Barthez and Carroll.
Back to the main point: we need a transformational keeper to make the leap into being a serious contender for the CL places*.
There aren't many around and we probably can't afford the likely fee - what do we think are the potential ceilings on Kovář or Vitek (and does anyone know what we might need to pay Leverkusen to bring Kovář back?)?
*Which may not be a big leap - none of Villa, Newcastle or Chelsea look like establishing themselves as a top 4 side on a regular basis...If we can deal with our major problem of simply not scoring goals and have someone who can make the difference in keeping them out in marginal games, well, who knows?