Well,you will never know the answer but i will try to answer:
1-A lot,i probably can’t give you a number,but someone after 1.7 had shown me the users of JWA,it have fall from this time.
If i remember correctly,before 1.9,the users have been divided by 2.
Sadly,i don’t know any programme which can say how much we are to play the game today and to know the real evolution.
In my country (France),All youtubers ,all forums and almost everyone have drop the game.
It became VERY rare to see a french people playing it .
2-Answer is a lot :But the issue don’t really come from health and damage boost,it had come from speed boost which have destroy the logic of the game ,you know “making a T-rex faster than a velociraptor”.
things have certainly been minimized with the introduction of boosts 2.0, but the game will not experience any development as long as we can exploit such an inconsistency.
In addition, the boosts were too long almost inaccessible (when you needed 100 boosts but you only earned 5 of each kind per day), things have been reduced, it’s less worse, you can get 10 per day, and 25 in a weekly incubator. But ludia took too much time to react properly and they lost a lot of clients.
Just on the forum, the two people I liked the most have deserted: Tuco and raven
3-Getting useless DNA have nothing really to do with boosts,that have to come with the balance of the game,which is a bit predictable (example of overboosted ardentis which get a huge nerf 1 update after,or buffing gemini to nerf his damage the patch after)or sometimes completly random (nerf apatosaurus ,ankylosaurus G2,koolabourgiana…)
But by experience,the sames DNA remain useless since the beginning of the game (All birds basically,deers,counter attackers,pterovexus …) while some others had their chance