Diversity as defense

Agreed, vendor lock in is a very big problem, what the economists would call increasing returns. Interestingly most of the research on the subjest finds that a vendor achieves "lock in" and a dominant market position not by being the most competitive product. Random historical accident, policies, market fluctuations, etc. - i.e. beta vs. vhs or CP/M vs. DOS vs. Apple. Probably getting far off topic here, but if you decreased the ability of vendors to lock in customers (increase competition) could you increase diversity and security at the macro scale.

Correct. Microsoft's problem is not security alone, but monoculture. If we
have all systems around Windows2003, we are exposed to risk of devastating
virus attack. No matter, how secure this Windows2003 is.