1. can the altcoins ever hit the ATH price even at $20000 bitcoin?
It is possible but this will take a long time to wait like years and year as we seen BTC did 10 years more to get that ATH.
2. Can the new coin from ICO be an alternative even with so much old coin held in some portfolios?
Yes ofcorse it is about the project they are offering, the people will decide on it in terms of demand.
3. will you choose to hold on to old coins or sell to invest into new ICOs?
It will depend on the market situation and the offered project. new coins are good to have if they can compete on the market and offering a great development for the future.