As for diversifying your portfolio, I don't think that's really necessary if you have full knowledge of Bitcoin and investing in it alone. As long as you understand everything about Bitcoin and the risks that are involved you won't really have much problem with it. Moreover altcoins hardly brings much profit as BTC, and most of them don't go up when Bitcoin is down, they usually follow the direction of Bitcoin.
But, I'm really against your idea, it's still good, everyone has what works best for them. Some of us don't really value altcoins, and there are those that values it. So, if you value them, you can still go for it and diversify your portfolio. But, remember no altcoin will be good for long term holding. Simply, altcoins are here for trading or else for short term investments.
As for investments, that's for sure that anyone that wants to invest in cryptocurrency should invest what they can afford to risk. These crypto assets are volatile and you can lose your money within a short time.
Talking of altcoins that will not be good for long term, I don’t think that we should generalize all altcoins for that.
There are some altcoins that as long as bitcoin exist, they will also exist, and those coins are the likes of Ethereum and some utility token, and also don’t forget that without exchanges, since we are yet to start using decentralized exchanges which I really don’t see people using, there will always be centralized exchanges like binance that will continue to keep up to expectation which you also know that they all have their utility token, so some of these altcoins will still stand with bitcoin in the future, juts that they will be very few and if I am to continue holding any altcoin after the next altcoins season, it will be Ethereum and any other utility token like Binance BNB.