For someone like me who only play 5 rift battles to keep rank and can slowly climb higher, this system totally destroy that. I'm sure many people will see the problems in that.
I share your concern because I was also only playing the minimum 5 up til end of rift 1.0. Wasn't one of the motivations for the rift rework to make it less of a stressful experience? Yes, by taking away the realtime head-to-head nature and the 2hr limit the per-battle experience is a littler easier, but on a weekly basis I don't see much hope of us keeping rank if we only play the minimum threshold (that is, unless you're an elite player or have extremely good luck with defense-building or matchmaking). IMO this puts the stress right back into the system for non-elite players.
That said, I understand rift is there to incentivize us to become more skilled and there probably isn't much of a place for more casual players in this game mode, but I wish there were. I also understand it's very difficult to find a middle-of-the-road solution.
Would it be possible to add a feature whereby we could see our realtime ranking in rift, e.g. if we're ranked 127 and there're 4000 rifters we'll see 127/4000) ? If we know where we currently sit we'll be able to better plan how much more effort to commit for the rest of the week. Obviously this is another stressor and may inflate the rank cutoff scores, so not sure it it's a good idea.