I don't think you have those reasons in proper order. I think the ability of the engineers to experiment and use new tools is why they have the high level of retention. I've seen plenty of places that pay much higher than the competition yet have very high turnover due to the a conservative culture which dictates the tools and doesn't encourage experimentation.
"An idle mind is a devil's workshop". I have seen this over and over again when you give engineers a free run. They micro-optimize things. They start rewriting javascript to c++ and c++ to c and asm when the speed improvements are at best marginal and the cost of maintenance of produced code is extremely high.
Don't get me wrong, I am not saying that these optimizations are bad. In fact, these engineers measure things properly and then rewrite. It's just that there is no business case for all this time spent. It's like writing web servers in C. Sure, it's possibly the faster than everything else but seriously? Who maintains all this.
