1. can the altcoins ever hit the ATH price even at $20000 bitcoin?
Yes. Bitcoin goes up, alts go up too.
2. Can the new coin from ICO be an alternative even with so much old coin held in some portfolios?
This depends on the investor and owner of a portfolio. If you think that its better to take new coins as an alternative to well established old coins, it's for you to decide.
3. will you choose to hold on to old coins or sell to invest into new ICOs?
Hold old established coins like bitcoin and eth and will invest with spare money if I see good project but I never invest to ICO. I'll invest once it is already established.
I humbly disagree with your first answer. I do not think any alt will hit the ath of bitcoin in the nearest future. If it happens, then am buying all in to alts.