I'll try and explain the reason that these devices are not compatible. Probably a waste of my time. However When you look at the technical side of things, there was not much choice to make an Expanded system while keeping the Compatibility the same.
It's a matter of how the data is transmitted through the Air to your handheld. there are allot of different ways to get a faster signal (More dogs), but they are all not compatible.
With the Astro you shouldn't think of the numbers as a Number 21.. Think of the number as two separate digits, For example 2 and 1. The First Digit of the Astro is the Frequency, the second is the channel. So ID number 11 is actually Frequency 1 channel 1.
Now fast forward to the ALPHA, You have twenty different Channels (Double the collars), but still 5 frequency's. Instead of having 119, (which would be Frequency 1 Channel 19) it was best that they did it differently than the ASTRO so not to confuse anyone into thinking 19 would work with an ASTRO collar 19. So you will see 1-19. Which means Frequency 1 Channel 19.
Although It would appear that the First 10 channels could be used the same however that is simply not the Case, The Two Units use a different Method (if you know what a Baud rate is, if not think Different Language) They have to do this so they don't mess each other up while hunting.
If some one was hunting with the Astro Collar while you used an Alpha collar, it would confuse the both of them and No One would be able to track anything. So Garmin had to do something to fix this, so folks were able to use an Astro next to an Alpha without causing a bunch of problems.
In order to track more dogs, The way Garmin did this was is to (somewhat) send the data Twice as fast as the Previous astro system, This way they could track twice as many dogs. Unfortunately when they do it this way there is no way to Track the slower ASTRO collars since they are not using the faster setting, It would get very complicated to try and figure out what collars to track.
Also It is a bit more complicated than this, but for the purpose of not getting into the weeds I've tried to keep it with simple explainations, However with the Training Portion of the System it adds some more complications to compatibility, however It could take quite a bit of Discussion and reasons and explaining All pointing to the reasons and the fact that the alpha is not compatible and It would be "EXTREMELY" difficult to make it compatible.
In the end, once you fully understand the technical problems, and the Large picture The Choice for Garmin was Either "NOT" Build a New system and continue with all the complaints about the ASTRO, or Build a new system that was not compatible. This allows you your choice, If you want compatible you'll have to stick with the ASTRO. If you want the Expanded version, and can accept the Non Compatibility then you'll move to the Alpha.
I have 14 years in The technical side, Sometime I take this information for granted when I look at something. I can see how folks that don't understand how computers and Technical things work would have a hard time accepting this.
If you have a better solution for Garmin, I'm sure they would love to hear it, And I would love to discuss it, I've got an idea to do it, but It's more expensive and Would not be compatible at all with Garmin, So my solution is really no better than The Alpha, In fact the Alpha is a better solution than what I could likely come up with.
In the Long Run, and in the Big Picture with all the information available, This was the right choice for Garmin to do. When they asked me about non-compatibility, I told them Better to do it sooner than later, and not discontinue the Astro.
In fact this was one of the only choices that Garmin had to expand and grow the Dog Tracking line.
To compare their devices to a telemetry collar with 10 simple components show's what Garmin is up against. I will hold Garmin Accountable when they mess up, (Like the DC40's, that was something that I could offer a solution for)
If I don't have a solution to offer, I normally try not to be a critic. I was much like you guys, until Garmin Asked me what Would I do. Then I had to sit down and think about it, so now I like to Think of what "I would Do" and that quickly brings things back to reality for me.
Now with that said, I'm sure Garmin will read this,.. so Please bring back the DC30's for the ASTRO! I still don't like the DC40s
Just my 2 cents.