I had the same experience, using Matlab in college in the early 2000s was great. But the upside of its decline is that it motivated me to pick up a more general systems programming language for scientific computing work.
The problem I have is that there is at least one Matlab toolbox I haven't been able to find an open source alternative. I use parts of the System Identification toolbox, but the parts I use aren't the parts that are implemented in the Octave one I found. There are tools in it that will do a transfer function estimate and various things with frequencies that I haven't found an alternative. I've bought the text that covers the toolbox and sometime when I have the time, I'll try and implement it.