I've been on both sides of the fence on this, so I feel qualified to answer.
I first saw idea that the stage was a Punch-Out stage on Little Mac's support thread. Someone had done a very detailed analysis of the stage and the elements that were similar (I think greenluigiman2 mentioned most of them). Prior to that, I was unaware that four ropes was indicative to a boxing ring. I had originally assumed that it was a wrestling ring since wrestling strikes me as more fitting Smash in general than does boxing. But all of the little details, like the style of the marquee lights, the realistically human audience...they all just fit TOO well with something that would come out of Punch-Out. I've played all the Punch-Out games and didn't realize it when I first saw it. But after someone else pointed the details out to me, I couldn't NOT see it anymore.
And I don't think it was a matter of them making it more obvious. I think it's more that they wanted to NOT make it TOO obvious initially before they wanted to announce him as a newcomer. I'm sure the design as it is now was always the plan, but doing that would have completely given it away.