I dont agree. I hold altcoins for long time and i know it is a good strategy. If you say it is a mistake that mean you are a newbie or you invest to shit scam altcoins.
Lets compare top a few digital currencies for 1 year ago and today.
Bitcoin - 1 year ago:~ 580 USD today:~ 4000 USD (x6.9)
Ethereum - 1 year ago:~ 11 USD today:~ 300 USD (x27)
Ripple - 1 year ago:~ 0.006 USD today:~ 0.29 USD (x48,3)
Litecoin - 1 year ago:~ 3.4 USD today:~ 47 USD (x13.8 )
Waves - 1 year ago:~ 0.16 USD today:~ 4.7 USD (x29.3)
As you can see if you hold known altcoins for last 1 year you would make much more profit comparing with holding bitcoin...
4 cherry picked examples do not make a convincing case. I'm disregarding BTC since that's obviously not an altcoin and I don't think anyone is disputing it's viability as a long term investment.
I would be more convinced if you took a sample of all coins on all exchanges. Then run the same time span comparison on them, and form a ratio of winners versus losers. If there's more winners than losers, I'll be... almost convinced.
The reason I say 'almost' there is this.
Let's say you have $1000 you put at day trading shitcoins.
And every day you fully commit that stack to your trades, and you make just 1% a day, and you cash out and the next day you do it again...
In a year you'll have
$38,181.
With the exception of Ripple, which before this week would have been more like 30x because it just now picked up, that's 38x and better than even the best performers you could find.
EDIT: Actually, Ripple is a terrible example. From 2015->2016, Ripple went from 7400 -> 1501... From 2016->2017, Ripple went from 1500 -> 650
Your argument is invalid.