I really don’t know how you conclude that Starmer is in any way an opportunist and even if he is, that he is somehow more of an opportunist than Johnson who has re-invented himself countless times to climb the ladder.
Brexit is totally illogical. Literally no good can realistically come from it when you take the emotion out of it. Johnson is desperate to drag Starmer and Labour into a culture war to pit them as the enemy of Brexit to stoke that emotion (Brexit and this Tory party need to constantly manufacture enemies) and he simply won’t allow him to do that.
The government has a huge majority and hundreds of MPs purely there as yes men specifically for Brexit. Nothing Starmer does or says can affect that so as much as you might wish to bleat about Starmer or random Labour/SNP/Lib politicians, they’re effectively powerless and if Brexit doesn’t deliver us all a glorious future with the streets paved in gold that’s on your party.