Then if adopted, will the Lightning Network's fees not be "unfairly cheap" forever?
Unlike on-chain transactions, off-chain transactions aren't limited in capacity, so market mechanisms should lead to an equilibrium between cost and profit. If fees go up a lot, new parties can join the LN-routing-market and fees will go down again until there's a new equilibrium.Quote
Is there some truth to what some people said that LN fees might be more than some altcoins' fees, because of limited capital, opportunity cost, and demand?
Yes 
An XLM transaction costs like 0.00001 XLM, which is about $0.0000008 or 5 msat (0.00000000005 BTC). There's no chance and no need for Bitcoin to compete with this, because it's Bitcoin, and not some altcoin.