Unclear that diversifying helps solve this sort of problem. More vendors could lead to the same number of bugs, but less investment in quality control per product e.g. if you make $1b and spend 1% on quality control, then you spend $10m checking your product for bugs. If the market fragments into 10 $100m vendors, then to get the same amount of money spent checking each chip for bugs, you'd have to spend 10x as much of your budget on quality control.
It gets even worse when you consider the incentives of private/academic/third party researchers to search for bugs, since there is a lot less prestige in catching a bug in a smaller/less well known product. e.g. I'm pretty sure no white-hat security researcher has checked for security problems in the cheap wifi light switches I bought on Amazon.