The 34-year-old lifted successive trophies a decade ago, and the Spanish driver appeared set to dominate the sport for years to come.
But that never materialized, as he returned to an increasingly uncompetitive Renault after an ill-fated move to McLaren, then had five years at Ferrari in which his racing reputation was widely praised -- but not rewarded in silverware.
Asked if he should have left the Italian team sooner, Alonso told CNN's The Circuit: "Probably, yes. The car was not competitive at all and things were getting more and more sad.
"So probably one or two years less was the best thing, but you know we tried to do our best and we fought until the end of every single race," he added.
"After five years in Ferrari, being second all the time, I think it was enough for me."
Alonso, now back at McLaren, sits 15th in this season's championship with just 11 points as the team's new engine supplier Honda struggles to get up to speed in its return to F1.
And yet, when he won the crown in 2005 and 2006
, so much was expected.
In 2007, the only season of his first spell at McLaren, he missed out on the title by a single point to Ferrari's Kimi Raikkonen but then returned to Renault after mutually agreeing to cancel his contract.
Finishing fifth and ninth in the following two seasons, he left for Maranello and was runner-up to Sebastian Vettel in his first year at the "Prancing Horse."
But the next three titles also went to the Red Bull star, who is now leading Ferrari's resurgence following Alonso's gamble that McLaren's fortunes will be resurrected by a renewed alliance with the Japanese manufacturer.
"I didn't want to give up and I wanted always to keep believing that it was possible and to keep the dream alive," added Alonso, as he reflected on his decision to quit Ferrari.
"Last year I realized that with the Mercedes domination it was not possible to win for Ferrari in the short term and a new project was the best idea.
"McLaren was a risky project because they were completely new, but we are one team -- we win and we lose together."
Alonso intimated his time at Ferrari hadn't been all that easy.
"With Ferrari sometimes, you win or you lose depending on what the mood of the team in general, of the group in general," he said.
"It's a very big team, with some good things, some bad things, and as I said it's good to experience and to live once being in Ferrari.
"But the most important thing is to win and to feel competitive -- and at the moment Mercedes are the guys that we need to beat," added Alonso, referring to the dominance of Lewis Hamilton and Nico Rosberg, who have won eight of this season's 10 races.
Much has changed in the decade since Alonso
became F1's youngest champion -- a record taken by a 23-year-old Vettel in 2010.
His 2005 triumph ended the dominance German driver Michael Schumacher, who had won five consecutive titles with Ferrari.
Now one of the elder statesmen of the sport, along with McLaren teammate Jenson Button, the 2009 world champion, Alonso has watched F1 evolve.
He recalls the days of the roaring V10 engines, refueling at pit-stops, and tires which could endure much longer than their modern replacements.
There is a sense that the enjoyment which was once so prevalent has begun to subside at a time where the sport has been hit by declining viewing figures and accusations that races have become boring.
"I'm sure that the grid has changed around 50% over the past two or three years so they don't know the Formula One that myself and Jenson and other drivers have experienced," Alonso said.
"There's the past cars that were maybe 10 seconds faster in our day, so when a young driver arrives in F1 now, they are surprised about the car -- but we are disappointed.
"We've been driving a similar car which is 10 seconds faster, so there are different opinions.
"It's not right or wrong -- it's just sometimes that you need to look for something you enjoy more."
This year has been difficult for Alonso -- especially after he missed the opening race following a preseason crash.
He suffered concussion and was airlifted to hospital after crashing during winter testing at Barcelona's Circuit de Catalunya on February 22.
He watched the Australian Grand Prix from his house in Abu Dhabi before returning to action in Malaysia a month later.
Next up, following the sport's summer break, is this weekend's Belgian Grand Prix, where Alonso is hoping to add to his career 32 race victories -- although he admits such a result would be something of a surprise.
"I think if the podium arrives, we'll not be prepared but it will not be a problem," he said.
"It will be a consequence of a good job and good progress in the car -- I think that's the main target for us."
Raikkonen to stay at Ferrari in 2016
Meanwhile, 2007 world champion Raikkonen will remain at Ferrari for the 2016 season, the manufacturer confirmed on its website, Tuesday.
The Finnish driver joined Ferrari in 2014 -- his second stint with the Italian team -- from Lotus F1.
"Scuderia Ferrari announces that it has renewed its technical and racing agreement with Kimi Raikkonen," they announced in a brief statement.
Raikkonen said to the website: "What can I say... For me, to be able to stay another year at Ferrari means that the dream goes on.
"The Scuderia is my family, as I always said, it's here I want to end my career. I am more committed than ever and I want to say thank you to the people who gave me this chance.
"Also, a big thank you goes to all my Ferrari fans, for their continuous support."