Phoebe Waller-Bridge did a great job in the film and that sort of talk smells like the usual Internet being garbage kind of content. The character of Helena Shaw actually makes for a great follow up to Mutt Williams. Whereas Mutt is a lot like a young Indiana, more headstrong and full of bravado with a plucky thirst for adventure, Helena acts as more of a foil to Indiana, opposing him in some of his most fundamental ideals, especially in his older age. Of course, since they're his adventure companions they both end up having hearts of gold, unsurprisingly. Helena is meant to oppose Indy, especially in the beginning stages of their re-united relationship.
I personally like the dynamic between Indy and Mutt better than the one between Indy and Helena but that's mainly due to the great (albeit predictable) twist that Mutt is his son that instantly changes Indy's attitude toward him. They also play off each extremely well and the call backs to Last Crusade except Indy is now in the father role are super fun (Indy repeating the 'Intolerable' lines, Mutt being proud of a stunt he's pulled only for Indy to look at him displeased, etc). Dial features similar moments but they just didn't hit as well, IMO, and it's more obtuse since she's not actually Indy's flesh and blood child, though the one call back to Crusade from Shaw was just superb.
With regards to 'replacing' Indy, I think both Crystal Skull and Dial both do a really good job at having these new characters introduced and figure prominently into the films without either of them feeling like they're jockeying for position with Indy or vying for his spot. They both handle those roles well. It is kind of shocking that they've resisted another character taking up the fedora or whip, even if only for a scene.
Harrison Ford is Indiana Jones. You don't make the movie without him. They will undoubtedly try in the future and who knows what the results will be but it will be a monumental uphill battle they have to fight right from the word go. I've seen talk of the idea that Indiana Jones should just be an American James Bond, a new actor every so often going on adventures. To me that whole idea scrapes against the core of Indy in the first place. He's a pastiche of era-appropriate adventure serials. The action in the franchise is what you would have seen in the 40s and 50s, just pumped up to 80s Hollywood blockbuster levels by masters of cinematic crafting.
That just brings me back around to the main problem I have with Dial: it's Indy in a modern action movie. It's set pieces are shot, edited and play like a modern action movie and, personally, it just doesn't work right with Indy. It's not bad, it's just not why I love the franchise.