Hey

Arena is getting naturally harder over time since people evolve their champions and adjust their strategies, especially in bronze or low silver it can be difficult to catch up.

Nevertheless Arena is in a constant flow and might seem harder or easier at times.

Cheers =)

or the higher scoring players are getting away from the game and their champs are just drifting lower and lower due to inactivity

This is happening yes, but inactive accounts do not show up for other players after they are inactive for a certain amount of time.

Therefore lower arena tiers aren't flooded with OP enemies over time.

I for one have about given up on Arena matches.  I managed to get up to L3 Bronze one time but then the competition got so hard I am back down to level 1.  NOW I get a progress challange to reach Silver 1.. LOL Fat chance of that ever happening.  It seems every one has a Kail at level 60 and I have no idea where they got him.  

I stopped playing for 7 monthes and dropped from G4 to B1.  It took me 5 days to get back to G4...  But in 3x3 I've spent 6 weeks struggling to get past Bronze 2/3.

Try to change and switch a little bit. I play on defence and dont get attacked more than 3x in bout 12 hours or so and in the offense i use similar tactic just with one (or 2) nukers with speed (as much you can get from the gear). I try to not give enemy time to attack my team,99% sucesfull if i pick carefully my matches. Hope this can give to you some ideas. Good Luck!

Oh and in tag team arena keep in your mind "not everything is gold what is shyning". I take sometimes champs what i know i possible can not defeat, but it turns out many times that they are not so "powerful" or they are not "puted together good as a team". I am in bronze 4 (even if i dont wish to be lol"

They are playing the game in the most efficient way - grind hard early on to level their starter to 60 within the first two weeks or so. You could do the same, then get a warmaiden and spirithost from campaign if you don't already have them, then you have a basic speed nuking team that would allow you to get a lot further.