Bitcoin isn't without weaknesses either. The biggest weakness for Bitcoin is China. China just own so much hashing power that if the Chinese government were to ban bitcoin mining/ stop it by cutting power to the biggest mining factories, bitcoin would stop functioning for weeks or months. The difficulty couldn't adjust to such huge changes.

Bitcoin is far from scalable and scalability is the biggest issue we have already witnessed. A 51% attack on bitcoin isn't something that we would expect to see, but then again we'd be surprised when it happens after attacks on the mining factories, which if I'm not wrong, control more than 50% of the hashing power.

So, I think that Bitcoin is not going to be the forever king of cryptocurrency and there is space for an altcoin to rise on top. But I agree with your analysis that the top altcoins also suffer from serious issues regarding blockchain security. But I mostly see that what will happen is not the end of altcoin era, but end of ICO/token era. They will disappear like shitty altcoins disappeared back in 2014-2015. And we've seen that starting to happen already, with 70% of all ICOs already dead, if I'm not mistaken.