For sure I cannot argue here and even having some groups configured (and using them for dedicated persistence strategies) I did not know of that rule trigger, maybe because I only wrote very little and simple rules so far. So thanks for opening my eyes.
Now I also found the DRY Design Pattern you have been referring to. Before I just googled for it and ended up on wikipedia.
For Influx the downsampling could probably be reached by a continuous query but doing the real filtering in SQL feels harder than doing it in the rule DSL. in MySQL/MariaDB stored procedures could be also used to cleanup. For the short-term (maybe an hour or day) high resolution storage it might be an interesting option, because it looks like RRD4J cannot be used for that and just deleting everything older than a day is not that hard in SQL 
So to summarize: it is quite some effort, none did it before with that level of complexity, to be portable the best is to do it in a rule with a trigger condition on a group.
Simpler solutions are described in
Another niche discussion was in