Explain to me how creating an alliance and moving back and forth between your own alliances is against the tos or unfair? Anyone can do that if they wish. Bhg taking steps to prevent the freedom to move to whatever alliance you want and preventing creating competitive alliances is a huge misstep on their part.
I don't see any reason why to explain a meaning on fair play, but ...
If a group of players has two or more alliances, with enough accounts to war all regularly, its fully ok.
If a bunch of idiots is running 5, 6 or 7 alliances by moving around its simply a sign of being assholes.
The one team you mentioned has warred two days before the decay, that's correct. But thins was the first war since October 2021 of that team. And immediately after this one war all moved out to other teams. May everybody think by himself what this behavior is, I say its douchebaggery.
Reading around the several forums, groups and servers your meaning is a minority one. Most players welcome the decay.
Plus, another huge benefit: since the decay matching is better, at least for our alliance. What is pretty logic as around 25 team were removed from top 100 and probably 50 from top 200. As matching must be related to glory somehow (even if there is no official statement) there are now more active teams in top 100 / 200 / 500 as before.
It definitely was a good move and seeing that some people are asking for the rules shows that some want to find a workaround to avoid decay.