[Windows7sins-translators] en/live/letter/ngos/letter.html: obscure wording

[Ineiev]

Hi;

This passage from letter/ngos/letter.html is not very easy to parse:

...charities and nonprofit organizations like yours
have relied on Microsoft and the Windows operating system, and because
they threaten to stop supporting older versions of Windows in the
long-term, and because their system is proprietary (not free/libre),
you are dependent on them to provide regular security updates and
fixes.

That is, "you are dependent on them"
"because they threaten to stop supporting older versions"

The logical link seems not very clear to me. probably this statement
connects to something else?  can anybody enlighten me on this?