It is absolutely true that big changes can (will) have unforeseen consequences, but they have been made in the past (15 to 13, changes to number of tackles, changes to numbers of subs). I'd argue the game needs that sort of change right now in order to survive, in terms of the concussion issue, and remain entertaining full stop.
Of course there are still great games of rugby around, but if you watch old matches to see how it was played in the 80s and 90s, I think it's a totally different level of excitement. I want to see varieties of passing moves, kicks, off-the-cuff running; all the unpredictable stuff that used to thrill me. A huge amount of that skill has gone out of the game now. I'm sure a large part of that is because defences are too fit and organised and there's not enough space/time for players to express themselves in any way other than raw power.
This is more urgent because the days of massive hits and fighting are gone and won't be back, so the game needs a selling point.
On the other hand I also know that I was a lot younger in the 80s and 90s so maybe that accounts for the added excitement of the era.