it is more reasonable when this could be optimally achieved when the coin is at a low price. The return will be so much and I don't think that anyone trusting Bitcoin for long-term investment will lose in the end.
Buying bitcoin gradually like you are saving fiat in bank saving is good. It helps you to restrict yourself against unnecessary spendings and increase total capital you spend to buy bitcoin.
"Will not lose in the end", I don't know but if they don't lose their bitcoin, they will probably get profit for long term investment.
Because it is a long term investment, they will have to know safely ways and have good practice to create their wallets, store their bitcoins, backups and protect their backups.
Leaving your coins on exchanges is bad but it is more terrible if you self storing your bitcoin and don't have backups or lose backups at the end.