Let’s keep this thread as a calm and rational discussion of the recent ENY changes.  Emotional outbursts and name calling can be posted on the other 12,000 threads. 
Not being a regular in EW, MW or AvA, I haven’t looked into ENY changes in these arenas, if any. 
Here is a summary of ENY changes between LW Tours 116 and 117. 
Model type   ENY -116     ENY - 117   Delta
A-20G   25   15   -10
Bf 109G-14   20   25   5
Bf 110C-4b   35   40   5
Brewster B-239   25   30   5
C.205   20   25   5
F4F-4   20   35   15
F4U-1D   15   10   -5
FM2   20   25   5
Fw 190A-8   31   25   -6
Fw 190F-8   20   25   5
Hurricane Mk IIC   10   12   2
I-16   30   35   5
Il-2   25   20   -5
Ki-61   25   30   5
Ki-84-Ia   15   20   5
La-5FN   20   25   5
La-7   5   8   3
LVTA4   40   35   -5
M-8   40   35   -5
Mosquito Mk VI   25   30   5
N1K2   5   8   3
P-38L   20   15   -5
P-39D   30   35   5
P-39Q   20   30   10
P-40E   30   35   5
P-47-D11   25   30   5
P-47-D25   22   25   3
P-47N   5   10   5
P-51D   8   5   -3
Spitfire Mk V   25   30   5
Ta 152H   5   10   5
Typhoon IB   10   8   -2
Wirbelwind   25   15   -10
Yak-9T   25   30   5
Yak-9U   20   23   3
 
[How does one get a table to display properly on this board?]
Everything in this OP is what I know of ENY.  Any errors will be vigorously pointed out by the community, I am sure.  
I have not seen published rationale for the values assigned to various aircraft.  There are two effects of ENY: restrictions on availability when the numbers of players per team is significantly unbalanced, and to calculate the value in perk points earned for different match ups.
The availability restrictions seem affect game play more often and more materially.  It is not unusual to see the ENY pop over 10.  A value of 20 is much less common.  I believe the maximum is 29, which is rarely seen.  Freakishly high ENY values are usually associated with off hours, arena changes, etc.
Is there any data out there on the time spent at various ENYs?  I am sure that more time is spent between 5 and 8 than 20 and 25.  How much effect is there in terms of time?   For example, how much more time will the P-51D be unavailable per month now that it is 5 instead of 8?
The fact that ENY values are used in the calculation of perk points earned really matters more to those who like to fly perked planes.  It seems that experienced guys are more likely to be motivated by this.  All things being equal, there should be a direct correlation between difficulty of scoring kills and ENY.
One idea that came and went in my mind was that of sliding ENYs.  The ENY of any given plane would go up and down based on how many were in the air for a country at any given time.  I haven’t thought out the actual formula.  There would be a base value which would change dynamically in response to the arena.  Maybe there could be a little lag to allow the big Pony hordes a chance to lift.  It would make re-arming a restricted plane more important and plane damage more significant as it would possibly require one to tower out and lose the restricted ride, at least for a little while.  This idea is not good as it would cause more grief to newbies as opposed to vets who would hold the rides longer on average.  
Another thought is to spend perk points for the difference between the ENY value of the airplane and the current arena ENY.  If you want a ride that badly, you can always pay for it.
I find it interesting which planes have been changed and in which directions.  A negative delta would mean HTC is reducing the incentive to fly a plane and increasing the value of shooting it down.  A positive delta is opposite, of course.  Apparently, HTC wants to see many more F4F-4s and less A-20Gs.
Personally, I have no strong feelings either way as I like planes like the C.205, Yak-9U and the I-16.  I rarely up perked planes, though I will grab a Tiger now and then.