That will never happen. Publishers put way too much stock in Metacritic scores these days and many get pissy with publications who score their games below an 8. The current scale allows sites to stay on the good side of publishers a little more without having to bend to their wills quite as much. That said, the likes of Square Enix and Nintendo have still been known to blacklist reviewers or send out “suggested changes” even with the current scale.It's less that 7/10 is considered rubbish and more like big publications moved their scale toward 9-10.
There's been a period (and it's still the case) in which anything decent was a 8, anything good was a 9 and anything great was a 10.
That's also the reason why Metacritic doesn't apply the same scale for movies, which a 61 grants you a green "generally favorable" score.
The day the press starts using a scale from 3 to 10 again is the day a 7 wont be average anymore but good.
For a change to happen it would take all of the big publications to make it in unison, but there’s no real incentive for them to do so as most people seem to accept things as they are. It would also effectively invalidate all of the scores given to games using the current scale, which is arguably far more problematic than the current situation.