Altcoin mining and altcoin trading is also a good investment. Most of them dont care about the altcoins but convert them to bitcoin and increase their stock of bitcoins. This is one thing that can get affected if altcoins cease to exist and the economy might get affected by it.
Again many altcoins are pump and dump schemes and the people should be careful of trading/investing in them.
I think before investing in an altcoin one should consider these thing,
1. not yet hyped or pumped
2. have good solid dedicated dev
3. have great roadmap
4. profitable in the long term
And I agree that bitcoin will be affected once altcoin ceased to exist because this altcoin simply gives a demand pressure to bitcoin because most altcoin can be bought only by BTC.
I'm not sure it's a good deal to mine those coins... They don't have the same stability, popularity and "tradition" of Bitcoin, making this kind of investment very risk.
You can start mining it today, and after 1-2 months, the coin price downs half of it price. So, you lost a lot of time and energy mining it and had loss in the end. I think it can worth if it's done as an experiment. Don't put a lot of money on it, just use part of your money to invest in alt-coins and alt-coin's mining.
Thats like bread without crust?
Or milk with curds?
or how about
a car but no gas!

Nahh, I think BTC could survive without altcoins, one thing doesn't fully need the another...
But it's good and helpful for traders and some gamblers make their profit.
