The return of Thor's hammer, Mjolnir, in Thor movies), Mjolnir's demise paved the way for a new era for Thor - one that could potentially be undercut by its return.
Mjolnir was destroyed in Thor: Ragnarok by Hela - who shattered it into pieces with her bare hands - but a video from the set of the Avengers: Endgame saw him reunited with the hammer. While this could make things confusing in of which weapon he now considers his primary one, it also highlights the potential risk of overreliance on a crutch that Thor has since moved on from.
The destruction of Mjolnir in Thor: Ragnarok was a powerful moment not only because it solidified Hela as a threat to be taken seriously, but it kicked off something of a journey of self-discovery for Thor. The Avenger had relied on the hammer to channel the lightning and aid him in all of his battles, so without it he felt helpless. However, it was only when it was taken away from him that he realized who he truly was. This was highlighted in that thrilling final battle sequence on Asgard when Thor managed to channel the lightning without any weapon, reminding viewers (and, more importantly, himself) that he was the God of Thunder - not the weapons that he wielded. Moreover, it showcased that he was worthy regardless of whether he had one to summon.
However, Avengers: Infinity War then saw the recently recharged Thor, fresh-off defeating Hela and an army of soldiers, unable to defeat Thanos in their first encounter before the hero subsequently spent a large portion of the movie forging a new weapon - one that seemingly made him even more powerful. With that in mind, it could be argued that the film essentially reverted Thor to his former self, rendering him powerless unless he was wielding a destructive weapon - something that was then proven when he came face-to-face with Thanos and later killed him with that axe.
Mjolnir's return in Avengers: Endgame did serve a purpose to the narrative as it proved to the despondent Thor that he was still worthy and his decision to bring it back to the present allowed Captain America to prove his worthiness too, but if Thor does rely on the weapon again, it would understandably leave viewers wondering what the point of its destruction was in the first place - especially when the character journey Thor embarked on has already been reversed through his creation of Stormbreaker.
Mjolnir's demise in Thor: Ragnarok was an impactful moment on many fronts, ending one era for the God of Thunder and starting a brand new one for him in the process. Though its return in Avengers: Endgame lent itself to his redemption arc nicely, the fact that it came after he already substituted it for another weapon - and that Mjolnir will return in Thor: Love and Thunder - undercuts that impact a great deal. And, in doing so, it risks regressing Thor to lesser version of himself.
