I would say that rank five is still somewhat meaningful, but will soon become obsolete.
However, just extending the grind to higher levels of mastery is only a short term solution as players eventually grind out the mastery even on champions they lose games with.
I think that champion mastery has the potential to become more meaningful if RIOT works out long term stability.
Higher ranks of mastery, in addition to requiring more grind, could be subject to some sort of decay.
The simplest form of decay would be not playing a champion that you achieved a high rank with.
If players keep their champion rank for years it would be like saying that since you learned a champion in season one, you still know the champion even though it may have gone through a major rework. It wouldn't make sense. So I suspect RIOT will implement something that decays champions from the higher ranks based on not playing them. And I also envision that the system will be ideal if players are forced to choose between maintaining the highest possible rank on a few champions vs maintaining a high rank on some champions vs a low rank on too many champions to maintain.
This only has importance if RIOT is looking at Champion Mastery as a way for players to show off their most played champions in the long term.
I say this because maybe RIOT has a different vision about what Champion Mastery means. In a vision, where Champion Mastery means only basic familiarity. RIOT can take a "collect them all" approach where players simply grind out the games necessary to get mastery and then players can show off that they aren't complete noobs at the champions they are playing (unless that champion was reworked, or nerfed, or bug fixed or "last season" or otherwise changed in one of those rare patches RIOT makes to the game).