You have asked many questions, i will try to answer each of them.
will this lead to an bubble? all those people investing and knowing nothing about crypto? don't know how to pay? how to deal with forks - or what forks even are?
Not really, no one throw his/her money in a pool just by seeing others doing the same, everyone who is in this business is well aware of cryptos and they know about it's worth and use.
will this together with the high transaction costs - and therefore no usability of bitcoin as a daily payment currency lead to an heavy decrease in value?
Transaction cost have reduced a lot as compared to past and it will going to improve more with Segwit and with the next big thing which is Lightning network, cure to all these problems. No usability are you sure? Many stores and restaurants are already accepting bitcoin as payment method specially some online shops.
and will the altcoins decrease as well as they are highly correlated to the bitcoin value? what is your opinion?
No doubt altcoins are somehow dependent on bitcoins and have to face some serious dumps in both rapid growth and fall of bitcoin but don't worry they will also shine if the bitcoin fly high.