No, Nintendo and Sega had no old systems out in 1986, because they were just getting started and had no previous hardware. As they both went on to show, as soon as they were old enough in the industry to have old hardware, they sold it and new hardware together, just like Atari did. Also, they were only in one market in 1986, while Atari already was a computer company, so they had computer stuff to sell too. As soon as Nintendo and Sega found another market (handhelds) they went in two markets too.
In 1986, Atari and Sega had only one new console each. By 1992, they are both supporting an old system, a new system, and a handheld. As soon as they were able to do what Atari did, they did it. It was not "being focused" that stopped them in 1986. It was having no options.
As for the computer comparison, have you looked at Atari computer games from that time? Computers had been well ahead of consoles (as they still are today). A slightly old computer in a new form factor actually was a really good console when the XEGS came out. Computers were Atari's biggest market at that time, and made them a lot of money. Your theory is that they should have completely ignored their biggest (possibly only) competitive advantage in favour of not doing what Nintendo and Sega weren't doing simply because they didn't have computers?