Alts have an effect by making the BTC more stable and have less/smaller spike effects. Simply because alts are traded against BTC and this generates a deeper market.
Some major alts like LTC and DOGE can also have small effect on BTC price, but not too much.
Only when you think of different crypto coins serving different purposes, it would lead to a higher overall market cap of crypto coins and therefore to a deepening of the market. By increasing the market cap of all crypto coins and introducing auto mining and trading mechanisms like the one of
www.genesis-mining.com the price of bitcoin could be stabilized. This mechanisms mine the most profitable altcoins and exchanged them to bitcoin. Introducing a perfect market

I'm I don't quite see what you mean here. Why would that lead to the stabilization of bitcoin? And why does miners hopping around drastically changing the hash on different networks help?