I'm in the opposite position; I've spent almost my entire five-year career focusing on one quite narrow tech stack and I'm good at it but I'm also sick of it and desperate to learn something new. I just changed jobs and I took a substantial pay cut for the chance to get professional experience in a new tech stack, which isn't ideal, but I really want to work with a new language and I don't know what else to do (there was no possibility of diversifying at my previous position.)

My point is that both the "short and fat" and "spikey" approaches have their pros and cons.