That definition for mainstream and extended support was certainly mangled.
Per Microsoft, the current policy provides 10 years of support (a minimum of 5 years Mainstream Support and a minimum of 5 years Extended Support). It's clear that mainstream and extended are consecutive, not concurrent. You need to be current service pack level to get the maximum support lifecycle. Older service packs expire 1 to 2 years after a subsequent service pack is released.
When mainstream support ends, you still get fixes for security and reliability issues, but you don't get new features or enhancements. Windows 7 with service pack 1 ends mainstream support January 13, 2015.
When extended support ends, you stop getting security and reliability patches. Extended support will end January 14, 2020.
So, Windows 7 still has a lot of life left in it. In fact, PCs with Windows 7 Professional can still be sold. End of sales for those PCs is October 31, 2016
I'm a firm advocate for maintaining currency with Operating Systems. Windows 10 (at the present) is the future. But, I don't necessarily advocate being on the bleeding edge. If you can benefit from the upgrade now go ahead. If I were running Windows 8, I would definitely upgrade now. But, don't feel forced. We've still got a while before the free upgrade offer expires. Hopefully, the initial hiccups and transition issues will be resolved by then.