about 7 years lifelong, execution by bullet... otherwise it gets too costly.
In OSM route_master is used for the lines. route relations are used for the variations, so for many lines that means at least 2 route relations in a route_master.
I've been proposing to use route_segment relations for the problem you'd like to see solved. One of the problems is that our editors don't have good support for that. I'm mostly mapping bus lines and it brings some complexity of its own. All in all it would also solve many problems. When a variation now gets broken, it's very likely all of them are, which causes a lot of work to solve them again, over and over again. If we had route_segments, we'd simply fix it in 2 route_segments and be done with it.