Let me give a specific example of why this arena will be at least -1 player as long as the airspawn alts remain fixed at 10k for every bout. I often duelled my Ki-84 against Blade's Spitfire Mark IX (compatible Timezone). I am on good terms with Blade who despite the language barrier is always civil and respectful and I likewise to him.
He's a Spitfire Mark IX 'specialist', and me a Ki-84 'specialist'. He's a very smart, thinking and adaptive adversary. When the fights were at mixed altitude airspawns the matches went back and forth a lot resulting in great challenge and engagement. Since the airspawns became fixed at 10k I noticed his ACM changed subtlety and I wasn't able to match him equally anymore. Consulting the charts I noticed the Spit9 enjoys a significant sustained climb-rate advantage over the Ki-84 in addition to a slightly higher top speed from there upwards for enough to count. The Spit9 has superior energy retention & more efficient energy exchange and a significant sustained turn advantage at all alts. I know Blade-san is an avid chart studier which is evident if you follow his blog.
The upshot of all this fine technical detail is that Blade-san has now adapted his ACM to exploit this 10k fixed airspawn and there's little I can do to stop him from gaining an energy advantage and then slapping me around with it. Many Japanese players favour this approach according to my observations. I can hardly ask him to reduce power or put on his stall limiter for my comfort & convenience. He is exploiting an environmental change. Fair enough.
It's not an issue of defeat - I care little for that otherwise I'd be campaigning for a fixed 6K merge, which would favour my plane. I'm not, I'm requesting the return to the more varied terrain both in terms of landscape and random altitudes which gave a fairer match-up over several bouts, balancing different plane attributes out and resulting in more variety of ACM. This would generalise to all sorts of combinations of aircraft.
I'm not sure why HTC sometimes makes changes because there's never notification. They do have consequences to players and if anyone is interested in hearing specifically why one of HTC's arenas has less players in it / has become less popular, here is at least one passionate data point to consider. Late Friday afternoons were once the highlight of my AH week. Now I don't log in at that time or go to the empty TA instead and practice AFM by myself. That is frustrating and sad.
I'm not being whiny, salty, bossy, demanding or threatening a dirty protest in the officer's club. I'm requesting respectfully for a more fair match-up for everyone in the Match Play Arena.
