There is so much hype about these currencies can this be a short term trend before it crashes?
Do not worry, you will see a lot of doom and gloom. It can be a short term trend or maybe not. ETH even if useful, is probably overheated. Bitcoin not that much yet, it grows at lower pace. All major altcoins are probably on bubble.
It is underrated by order(s) of magnitude still, so it can go up a lot. The pace is a question and then the other economical factrs as well.
Can these trends be sustained?
Not sure what trends you talk about. If it is hype and crash then yes, it was here all the time and will be here with so mispriced assets in future. They will always grow and then the traders cash the profits and crashes happen. This will happen even in future until the price will be closer to the reality that we live in. The reality we are starting to live in is - that in cashless economy huge part of human production will be possible to trade only through cryptocurrencies.
What needs to happen for these currencies to really take off?
They need to grow to the point where they will be lucrative to institutional investors. It is still not there.
My advice is just take it easy. There are no surebets. You turned from crypto-hater to crypto-lover I think. It is common. It is normal after all. But there are o surebets, there will be times when maybe overnight any cryptocurrency will lose tens of percents, even when things go well. And they will not be smooth. Are you ready for that?
If you talk to God, you are praying; If God talks to you, you have schizophrenia.