Price of an altcoin is determined by the demand: people that wants that altcoin would buy it with BTC, raising its value. Instead now there's more people that's simply mining that altcoin, with less people buying it.
That won't be the case. The trading will still continue. All thats going to change is the initial value of new altcoins.
People will still sell and buy altcoins for BTC.
True. Eventually all alt coins are sold for BTC (and then for fiat).
As far as betarigs influence, well, the hashing power of all these rigs is out there, with or without betarigs. This is just a marketplace where rig owners list their hardware instead of mining themselves.