I agree with everything up until Goku's level after the ki blasts. Doesn't it seem strange that a generic ki barrage would drain more than half of Goku's remaining power, but a fully charged KHH only drained 20%?
Vegeta also was absolutely shocked when Goku gave up, even though he knew Goku wasn't going to take a senzu bean. It looked to me that in Vegeta's eyes Goku was still their best bet. I think Goku experienced a continual drain of power after he lost his fighting aura. MSSJ was meant to keep the strain of SSJ at a minimum, but after the fight Goku had I believe it was inevitable for his power to continue draining until he became weaker than even Piccolo. He definitely wasn't portrayed as being that weak when he gave up the fight.
I think considering the nature of the effort and intensity of the chi blasts, with Goku spamming it so much with no thought, forcing Cell to even put up a barrier, that such was like Goku's last effort in dishing out powerful blasts as his last attempt in the fight. He knew he couldn't win already so he gave it everything with those chi blasts throwing them over and over, which, to an already drained warrior, can be too taxing, and as we saw, Goku was breathing way too heavily after that. I don't think the energy loss was continuous after that... I think Goku's condition after that last huge effort was just that bad... He returned to his no-aura, SSJ resting state, which can exist almost like a base state, so I don't see signs there of continual loss of energy when he wasn't exerting himself. It's different from Goku losing even more power when he tried to gather energy for one minute with SSJ3 for example, because there Goku was exerting himself in a state he wasn't used to. Here, he was moreso with the relaxed state not exerting himself... I don't see means for a continual power loss, not when he wasn't in fighting mode, or had his power up, or anything. My opinion, though.
As for Vegeta, I think it's different because Vegeta acknowledged Goku couldn't win... So Trunks was with the same kinda question like "but then... What do we do..." and Vegeta pointed at Goku having a plan like they had deduced earlier and to pin their hopes on that. He knew that power wise Goku was lost... He just thought Goku still had a reason to keep that calm demeanor he had always maintained, since before the CGs, so he was expecting something new there... what, he himself didn't know... But that works whether Goku is above or way below him I'd say, because the unexpected thing he was waiting Goku to pull was independent from the power Goku had at that moment, hence their surprise when Goku gave up, like "what... So is that really it? He hadn't a plan all along?" ... It boiled a lot down to "what Goku had up his sleeve", from the way I see it.