You crack me up. Always so angry and opinionated.
And I'm assuming you'll also say that pro's never picked Sven as a support. Just because someone can be a carry, doesn't mean that can't also be a support if played as one. Pro's don't pick Alch for support because there are usually better ones. That doesn't mean alch CAN'T support. It just means when you CAN CHOOSE YOUR HEROES, you probably wouldn't pick him. But if you do an AR and get alch and 4 other carries, you could play alch as a support in that situation.
For someone who is constantly on the case of "Omg everyone just follows what the pro's do even though there's so many other viable playstyles", I have no clue how you can use the "Why don't pros do it then" as an argument. You do realize those two arguments are fundamentally opposed, right?
Alch has a very long duration and potent stun (though yes it does have situational issues, but so does Lina and Leshrac). Maybe roam him with another stunner who initiates first. Alch has acid spray which is great for pushing as it owns creep waves and also makes it hard for enemy team to counter your push (especially at early levels). Acid spray also reduces armor, solid for a teamfight. Alch is also good at stacking neutrals, especially in radiant jungle. And then can clear them all with well placed acid spray. His greed increases the gold he would get from this jungling and allow him to buy items quicker than other supports would since they can't clear the jungle (with exceptions) and dont get bonus gold. Finally, his ult is solid for move speed and regens.
Is he an ideal support, probably not. Is he better as a carry, probably. But just because he doesn't fit your predetermined notion of a support, doesn't make him useless as one.