Exactly, so as time goes on there will be more younger people reaching voting age who will come to realise that they have been cheated out of the rights and freedoms that EU citizens enjoy and that previous generations in the UK, including their parents' generation, took for granted. Brexiteers might argue that those born from the early to mid 2000s onwards will only ever have known life outside the EU at least in their adult years so they will come to regard it as the norm, but in many cases their parents will tell them how much better life was inside the EU in the 1990s and early 2000s.They may have their personal views but there is a delicate balance to be made so the safe option is just to cut and paste the party line. In particular, many Labour MPs and members are likely to favour in principle a closer association with the EU going well beyond what is currently proposed. But this needs the EU to be interested in doing so, and confident that whatever is done won't just be reversed by a future Tory government. So expressing that sort of view risks unreasonably raising the hopes of those who share it, and annoying those who don't - but also the many who remember the strife of 2016-24 and don't want to go back there.
Over time, more Leave voters will see the downsides of Brexit and the demographic will shift as older and more likely anti-EU voters die off and younger more likely pro-EU people reach voting age. That process has to run for quite a while more before there's any question of a major realignment towards the EU.
As the electorate becomes more pro-EU as more younger people join the electorate and more older Eurosceptics leave the world, it will become more difficult for anyone to argue that the very close result of a non-legally binding referendum must be respected forever or that the government must continue to kowtow to a vociferous and diminishing minority who consider themselves entitled to have their way for the rest of their lives regardless of whether they were aged 18 or 80 in 2016.
Granted, the pro-EU pamphlet that was sent to every home in the UK did promise that the referendum would be "a once in a lifetime decision" and that "the government will implement what you decide". But technically David Cameron was overstepping his authority by saying that because governments and parliaments may not bind their successors. As such, neither Cameron nor any of his successors (including Starmer) have the power to compel all future governments to abide by the 2016 referendum result. Also, Cameron had told parliament that the referendum was to be advisory only and would not compel the government to abide by the result.