How wrong they were.
Not only did Ennis-Hill banish any thoughts of retirement by returning to the gruelling heptathlon with the same desire that drove her to London 2012 glory, she reconquered the world a year after giving birth to son Reggie and then, in Rio, went on to win her second Olympic medal, a silver.
She shed tears of joy, relief and exhaustion at the conclusion of her seven events in Brazil and admitted that she had a decision to make over her future.
Sixty-one days after winning silver, on her own terms, Ennis-Hill finally made “one of the toughest decisions” of her life.
It was time to retire.
Unsurprisingly, like any new parent, Reggie had changed Ennis-Hill’s priorities. She was a mother first and an athlete second.
But what an athlete she remained. Not to mention an ambassador for athletics and an inspiration for women and young people in particular to take up sport.
The sight of her crossing the line - arms outstretched, face raised to the heavens - in the final event of the heptathlon, the 800 metres, at London 2012 was one of the great moments in British sporting history. The poster girl of the Games had lived up to all the hype.
Beginning at the City of Sheffield and Dearne Athletics Club as an 11-year-old, and coached by Toni Minichiello since the age of 13, as an up-and-coming athlete Ennis-Hill was nicknamed ‘Tadpole’ by compatriot and fellow heptathlete Kelly Sotherton.
It was a barely-veiled dig at the young pretender’s petite stature. And it was a tag to which she would prove laughably unsuited.
At just 5ft 5in she has been towered over by her rivals for her entire career.
But, for an athlete wrought from pure Sheffield steel, it mattered not one bit.
Ennis-Hill’s first senior international medal came at the 2006 Commonwealth Games in Melbourne.
She suffered a major setback in 2008, though. Three stress fractures in her right ankle not only ruled her out of the Beijing Games, but threatened her whole career.
Her response epitomised the bulldog spirit and relentless drive she has become known for throughout her career. The next year she was world champion.
She won the world title indoors and European title outdoors in 2010 before having to settle for silver behind Russia’s Tatyana Chernova at the 2011 World Championships in Daegu. Ennis-Hill is still waiting to see if she will be upgraded to gold after her rival was exposed as a drug cheat.
It is the subject of Chernova and the lengthy fight for the medal she believes is rightfully hers that gets the normally cool, calm and composed Ennis-Hill closest to anger.
She missed the 2013 worlds with an Achilles injury, marrying husband Andy in May of that year, while 2014 was given over to maternity leave.
Any suggestion that motherhood would dim Ennis-Hill’s competitive instincts were dismissed by the woman herself, but the speed of her return to the top took even her by surprise.
She won gold at the 2015 World Championships in Beijing, just 13 months after Reggie’s birth and a mere 15 weeks after she first dipped her toe back into the waters of competition.
The 30-year-old will not be seen in competitive action again. But the amazing memories will live on - for her and those who marvelled at her remarkable ability.
She is truly one of Britain’s all-time greats.