Coaching is probably something that is time limited in terms of effectiveness. Even exceptions like Alex F for Man U made coaching changes to keep it fresh
Recent rugby examples, Mike Ford went from a very successful international defence coach career to Bath, in 2011, became head coach in 2013 and after some good seasons including losing the the EAs in a Prem Final, left in 2016 after results downturn - so about 5 years in his case.
Jim Mallinder went to Saints in 2017, he went really well to 2014/15, and then his decline was hidden by previous good will until being sacked three years later, you could argue his "success" period was 7 years long - quite long in coaching terms
At Wasps, one of the best Geech lasted 5 seasons, Edwards was Head for 6 seasons, Gatland 5 seasons, I do think that 5 or 6 years is about a coaching "cycle"
At Tigers - Murphy has been there 6 years and they have had a mini peak with the Anglo-Welsh and now a decline - now looking to Ford as new blood but problems continue (though that might be not connected to rugby?)
Dai was at Cardiff for 9 years, arguably a couple too many and has been with us for 8. In terms of coaching cycles, that might be about the right time for a change?
Obviously - I've picked examples to make my "case" you could point to the EA's but I guess having a first team pack with 5 England starters makes success easier -