Dec fronted this year's live shows alone after long-time co-host Ant stepped down from TV work.
Although the pair were seen together for the auditions, Dec went solo for the live semi-finals last week and won huge support from viewers.
Boss Simon has also been impressed, saying at the weekend: “Dec has been unbelievable. I saw his nerves in the first five minutes of the live shows.
“I could feel it for him. The night the lightning struck and we went off air, I made a quip about Ant – and in a weird way it just cleared the air. From that moment on Dec has owned the show.
“I’ve tremendous respect for him. He’s been a revelation.”
But Simon says he fully plans for Ant to return to the show next year, following a stint in rehab after a conviction for drink-driving.
In an interview with the Daily Mirror, Simon also touched on Ant's split from wife Lisa Armstrong, who works as a make up artist on BGT.
Simon said: “He’s not a bad person, he’s not the only person these things happen to. Lisa is still working on the show. She’s a great girl, I felt for her.
“Ant has taken a step back because his health is way more important. I’m almost certain he’ll be back next year – and we want him back.”
Meanwhile, following last night's BGT final, viewers have Tweeted their praise for Dec.
"Can we just take a second to appreciate what an absolute legend Declan Donnelly is! He’s done so well by himself 👏🏼 #BGTfinal," one wrote.
"Whoever comes out on top tonight, I think the real winner of #BGT this year is Declan Donnelly. What a star," added another.
A third posted: "Ladies and gentlemen the winner of bgt 2018 is Declan Donnelly. What an opening."
Britain's Got Talent airs on ITV.