Could we design it so you can’t reply to the Topic more than X times but this is not enforced if you reply to individual posts explicitly using the individual posts’ reply buttons?
That would be a good compromise, no?
Part of the justification would be that splitting up a single post by quoting individuals is a lot more complex a task for a new user than simply hitting the reply button of each post you wish to respond to. It’s also much more tricky to quote from multiple posts on mobile.
I see many justified cases of users replying individually to several different people in a row.