I agree that in the long term, the democratic party would benefit from having younger politicians in their caucus.
However, they also have to consider the short term: taking back as many seats in congress. This may be a case where those 2 "priorities" are in conflict.
Unfortunately, unseating an incumbent like Collins is not an easy thing to do. You point to Platner and his "qualities" that would make him a good candidate. But Mills 1) has more political experience, so she has an idea how to speak to crowds, how to debate, etc., 2) Has much better name recognition, 3) probably has all their "skeletons" exposed already.
The thing is the Dems have been fielding plenty of Mills' over the last few decades and they either turn out to be pathetically useless (a big part of the reason Collins is a senator despite being a TACO suck up who occasionally voices "concerns") or turn out to be turncoats who actively destroy the few good policies the party champion (think Sinema and Manchin).
Before you criticize Manchin, keep in mind that he is a senator representing a deep-red state, that he probably only won because of his history in the state. I suspect that any other democrat (regardless of how young and dynamic they are) probably would have lost the state. Yeah, it sucks that he's not pushing back harder against republicans, but its the nature of the voting base in the state. And even though he's not as "left wing" as most people like, he is still closer to the average democrat's sensibility than any potential republican replacement. He voted to confirm Jackson on the supreme court, and he voted for the Inflation reduction act. (Given the widespread opposition within the GOP, it is likely that had the republicans held his seat, they would have voted against it.)
As for Sinema... you cannot claim she is some sort of "old centrist installed by leadership against the wishes of a left wing electorate" because 1) she was under 50, 2) she had the markings of a more "left wing" democrat (including spending part of her earlier career associated with the green party, and 3) said she would not support Schumer in a vote for party leadership. In other words, she checked all the boxes of what you seem to want in a party candidate. The fact that she ended up being an obstructionist seemed to come out of left field.
The simple fact of the matter is that the Democratic party's natural voter base is (at least in economic terms) significantly to the left of the party leadership
Is the base really "to the left"?
People vote for a lot of reasons... some economic, some on social issues, some even on "fake" issues (i.e. not so much on reality but on what they are convinced is true.) Unlike the cult of the GOP, to me it seems like the Democrats have to manage a much wider group of voters...the "leftists" who want social programs, the environmentalists, minority and LGBTQ groups wanting better social justice in society, women wanting reproductive rights. Its like herding cats. And there is no guarantee that trying to appease voters who want one thing is not going to negatively affect voters who want OTHER things.
Hence why the party has been losing its core vote. Taking your core for granted and chasing after your opposition's voters by mimicking your opposition's policies is a losing strategy, yet the Dems have been doing it since at least the 90's and continue today.
That's the claim... "centrism is driving extreme people away". But I think the reality is a lot more nuanced.
At least some of the "far left" are going to hopefully be smart enough to recognize that, despite the democratic leadership being more moderate than they want, voting for them is still better than the republican alternative. Plus, in any election, not only do you need to get people to vote for YOUR party, you also need to prevent people from voting for the other party. Catering to only those on the far left will not only lose the centrist votes (something you seem to be willing to do), but will also cause republicans to become more energized because they will see the need to stop "the radicals".
Oh, and by the way, remember this: From the 2000 presidential election and beyond, Republicans have only won a solid majority the popular vote once (that was under Bush.) Other times, Democrats have either won the election outright (Obama and Biden), won the popular vote but lost the electoral college (Gore and Clinton), or at least kept the republican from winning a majority of the popular vote (Harris). Pretty good for a "losing strategy".
The biggest problem for the Democrats is that politics are rigged against them... the electoral college favors idiots in largely rural (red) states, a lack of ethics on the part of republicans leads to gerrymandering, voter suppression, and the current supreme court (the best that money can buy), and pro-fascist news outlets like Fox News and Sinclair broadcasting do not have a left-wing equivalent.