That's the sad truth. Diversification is a good strategy but people forget that Bitcoin itself represents financial diversification. The destiny of cryptocurrency will hinge solely on the fate of Bitcoin for the foreseeable future, so all in on Bitcoin is the way to go as a diversification into crypto.
It's consistently the best performer, and the least affected in bear markets like now. Any gamble on alts, as you say, is short term. You are as likely to lose as to win. Alt speculation is just that, speculation. I see I'm not the only one who sees new alts as ultimately that alone.
I think some established alts worth attention from the perspective of diversification because they have already showed some real progress. For example, I really like Monero, because it probably has the best privacy amongst all coins, and it shows a very strong long-term pattern for now, but even it is not perfect, I feel like it is very dependent on its legality. If I'm not mistaken, Monero has reached ATH when some big Korean exchange has listed it, but now it's sinking as governments are telling exchanges to delist privacy coins. Ethereum also shows a good long-term potential for at least a couple of years, but it might eventually lose to Bitcoin if Bitcoin will get smart contracts and other features, and if PoS will eventually fail as some experts predict. So, even with some solid altcoins its hard to diversify, so crypto portfolios should have 85-90% of Bitcoin, especially during bear markets.