Ask HN: Is choosing a right stack for startup really important decision?

When I have an product idea to develop, I generally mold my ideas which fits best with certain type of stack (LAMP, MEAN, Djano etc). I feel that certain things are best when its done using certain stacks.

Like for example if we are looking into real time chat application its best to use MEAN stack, but seeing "Slack" which is completely built on PHP and MySQL makes me think that choosing the right stack does not matter because at the end anything can be scaled

I reside in Dubai and the developers and startups do not have a clear understanding of my question and I don't know whom to ask..

Is it really important spending lot of time in learning the MEAN stack if say I want to develop a realtime sharing/messaging application instead in PHP and MySQL?