In my experience the change is BIG.
I started playing in december, and with a F2P Fast team i was able to reach G1 by february...
After a couple of months without playing, i returned the first days of may, and i was in B1...
When i tried Arena again... that was simply an impossible task, all matchups where really hard, full legends 60 with arbiter... or other top tier champions. I manage to speed up my team to 250+ speed (for B1 arena team should be more than enought...), but just be able to win 1 or 2 battles in a row. Slowly, i reached B4.
Last week of may however, bots start appear... and suddenly in a couple of days I jump from B4 to G2 (almost G3, just a couple of wins left). I have made some changes and upgrades to my team, but bots matchups make the difference.
Besides, here in G2-G3, im finding more winnable matchups than in B1 - B2.
I dont know if this idea with bots is the better solution to Arenas, but at least it allows early players to grow there and get the medals to can advance in GH and complete the arena progession missions´s.