It seems to me the most straightforward approach from a user standpoint would be for a quoted reply to default to a display of maybe four or five lines of the quoted post, which would be in view automatically. Then, with a 'Show more/less' toggle, a viewer could make their own choice of whether to look at the full text or not.
Anyone wanting to focus attention on a particular point they are responding to would be sensible to make sure they edited the quoted text so that it featured in the top few lines of their reply.
People who want to read quoted posts in detail still have the option right there for them. People who want to skim through aren't overloaded with long quoted text. Everybody wins.
Of course, it is easy to talk about things that it would be 'nice to have'. To actually have it, somebody else is going to have to find an efficient way of putting it into practice ... and we know who that is, so I'm not going to bang on too much about this. I'm grateful for what we already get.
David.
Comment and (constructive) criticism always welcome.