a) that has not been stated anywhere.
Notch clearly stated that 1.8 uses new terrain generation method (thus we will see things such as complex dungeons and rivers) that people won't be able to load their old world. You may want to search for it.
b) Even if the saves are no longer compatible, that does nothing to change the damage we'd be doing by changing the save format.
For example, a new player starts playing Minecraft post 1.8 with a new world. He then decides to eventually start playing mods and installs Forge with MCE-style functionality. His save game is then permanently altered and he can never stop using the Forge if he wants to continue playing it.
How does 1.8 potentially including a new save format change the problem in the least?
Yes, but those are mods, not an API. Just installing a tool should really not alter your save-game format IMO.
Also, new blocks and items can be deleted from a save relatively easily if you decide to stop using a mod. If the whole blockID system is altered, that can not be so easily reversed.
I am not sure if I am following your definition of API, but I believe it is rather unavoidable step that Forge should be. If a person who has been using the Forge decides to stop using the Forge, then the only logical reason he/she does not want to use it is because he/she does not want to use the mods that are using the Forge, for either.....
1.) He wants to play complete vMC experience.
2.) He wants to play MC with mods that are not using AND incompatible to the Forge.
I've already covered the first point; even though current system allows people to simply remove the mods from the game, if they have already invested deeply on modded blocks (which it WILL happen due to the nature of the mods using the Forge in general), a mere removal of the mod will cause the whole world being unusable. Like....
You will see a big empty tower that does nothing (formally pulley elevator)
Ugly shapes of ground (formally pipes were there.)
And a lot of active mechanisms that are depended on mod blocks will be useless, and the work to be done to make them usable again with vMC setting will take a long time to accomplish if not impossible. For the most people, it is logical that they should start new fresh world.
For second point, the person who decides to play MC with mods that are not friendly with the Forge will face similar concerns plus more. Like first case, a lot of things will be broken and useless without the mods using the Forge that simply restarting the new world becomes the most desirable choice. Not to mention, as far as I see it, most of modders using the Forge currently do not have alternative version that are not using the Forge (well, but it is obvious that supporting non-Forge version only weakens the Forge's position in minecraft modding community).
Even if a person has been using the mods that have non-Forge version, he/she will still have to overcome issues like class file conflict issues low block id issue and other very nasty things.
Without the Forge related, it is common issue and only solution is usually starting new world. When Aether and BTW were semi-compatible. I did a lot of stuffs with Aether blocks and BTW ones. When I heard about the Forge and Flower going for it, I had to abandon either Aether or BTW. Obviously I chose BTW, and I found out that most system just does not work anymore without Aether blocks and error with Aether only features.
That alone I had to just abandon that world (good thing I had two versions of my main world. One with Aether and one without.). As you see, even before we start discuss about the Forge API altering save files, in current situation, there is no other way but restart new fresh world anyway in order to solve the problems from the mods, making the whole point 'unable to load the save file with vMC' irreverent, because people will 'be never able to'/'do not want to' load save files with vMC in the first place, regardless of the save file corruption.
At last, back to the Forge API itself again, I understand it as like DirectX for games in windows. You have to install it to play games (mods in minecraft) anyway, and it should be installed in windows (minecraft) copies as many as possible. If the Forge API becomes popular (it should and will), the save file issue will be non-issue.
Pffft. I put absolutely no stock in the "beta" excuse. Mojang does not follow a designation system for such things even vaguely resembling any other game development studio.
In my eyes, the game was released to paying customers long ago. It just happens to have a rather active patch and update schedule. They can call it "gamma" for all I care. It really means nothing.
Maybe, but also with 'beta' tag they can try something like 1.8 otherwise a lot of people will rage when their old world does not work anymore :P
Plus : Real issue is indeed performance and multiplayer.... But again, it is not unknown fact that huge mods in general strain old computers a lot in the first place, and multiplayer coding on minecraft is really, at best, incomplete.