Defoe made an emotional return to the club on deadline day in January but retired in March.
Sunderland's striking depth was threadbare in the closing weeks of the season after Nathan Broadhead suffered an injury, but the club's Sporting Director says the club made the best decision at the time.
He said the hunger and motivation in the dressing room to get over the line was a key factor in the club's eventual promotion.
“Jermain came to that conversation full of optimism on what he felt he could come and provide," Speakman said.
"The environment in the changing room is one of the components in the changing room that has got us over the line and what you don’t want in that type of scenario is people who are not committed.
“If you look round, I was really confident of the play-offs because I could see a group of individuals that had the bit between their teeth and they wanted to be the group that got Sunderland back into the Championship.
“I don’t think within that, you can have people that are bit-parts. I think Jermain made a really tough decision that he felt that he couldn’t continue. It’s not ideal because naturally you lose a player out of your squad and whether that is through injury or something else then that can create an issue.
“I don’t necessarily look back on it and say we should or shouldn’t have done it. I just think that we make the best possible decision we can as a club that, given time, sometimes they work or don’t work.
“Did this one have more risk? Of course it did. Because of age profile, playing history etc. But at the same time, there will be other risks that we taken which were really successful that we don’t talk about because they have been successful. Therefore, it is what it is."
Sunderland are eager to bring Nathan Broadhead back to the club next season, and will also step up talks with Ross Stewart over a new long-term contract.