it is a development machine with highly specialised tools - Altium Designer, SolidWorks, IDA Pro, Altera Quartus, etc.
Upgrading the OS is not a trivial thing as would be on a phone or tablet. Also when upgrading the OS it would make sense to upgrade the HW as well, and that is a major investment. And Signal is just not important really to warrant that.
I would still use it on my phone though, but on the PC is just Viber unfortunately (whatsapp dropped as well).
Yeah, he was always that bad. There is a an excellent example in Eric Berger’s book about SpaceX early days(Lift off). I don’t remember the exact details, but there is a critical failure with one of the rockets, and Elon blames it on one of the best engineers. Later is found out it was not his fault, but Elon never apologised which just leaves vibes in the company.
And overall feel of reading the book that Elon treats everyone kinda bad, it is just not directly stated.