Stash space is not big issue for season real in S6 as players only keep gears relevant to the build. It will be an issue in S7 when Armory is introduced, players may want to make 5 builds per class to utilize the maximum slots. The current stash space will be insufficient to support the gears for additional builds plus boss mats and rune words.
You can extend your stash space with alts, but it will make it an inventory management game. You will have to keep a journal of which alt is responsible for keeping what type of gears. Whenever you get a better gear, you have to login to the correct alt to compare the gears before you salvage the old one.
As gears and runes are expensive, you also need to keep more than one alt to keep the gears you plan to sell. As those gears may take a while to get buyers, especially you don’t want to sell them cheap.
With all this inventory issues, I doubt anyone can run two alts with 10 different builds in S7. Maybe you think you just stick with 2 builds to avoid stash issues, then you may not enjoy the new features in S7 to its full extent.