Altcoins is not something you can just predict and plan for long term cause most of them won't last long at all
A coin can look strong and still lose hodlers overtime
I wouldn't recommend planning for a long term hold on an altcoin
Most altcoin won't survive in the long run, but we shouldn't lump them all together. ETH has been around since 2015, while LTC, XRP, and XMR have been around since around 2011-2012, and many other altcoin will continue to survive and grow alongside Bitcoin.
Long term, Bitcoin will always be the best choice. But that doesn't mean no altcoin is worth holding long term either. However, picking the right altcoin is very difficult, and everyone should do their own research and make their own decision. I don't want to give any specific advice, because there's no guarantee when it comes to altcoin.