You could make the same argument when it comes to other politicians who "grew" as they became older/wiser. Hell, FDR and Winston Churchill could conceivably fit this paradigm (and no, I'm not saying Kamala Harris is FDR or Winston Churchill or will ever be half of what they were), just that FDR and Churchill were considered "empty suits" by many before they rose to the top.
Bill Clinton gave off this vibe in the years before the 92 campaign. Trying too hard, not comfortable in their own skin before the right time came along.
We'll see with Kamala. In any event, she's not-Trump, so that makes her good enough per the 24 election. Her Presidency will determine what happens in 28.
Um, sure she could turn into the greatest thing ever overnight. That's not the question. The question is why she sucks now.
That said, I think your comparison to Clinton is incredibly absurd. Clinton was always super comfortable in his own skin, and was an A+++ communicator from his first moment on the scene. He was unquestionably smart, and impressed everyone he came in contact with, and created intense personal loyalty with his empathy.
He had some competency issues, and a lot of missteps, and was absolutely ethically dubious at all times. But comparing the communication ability and intelligence and people skills of Kamala Harris to Bill Clinton is like comparing a high school bench warmer to Michael Jordan. Clinton is in a very small handful of national politicians in excelling in those areas, among the best that ever was.
Bill Clinton also very famously and deliberately ran from the middle in contrast to where Democrats had been in the previous cycles, which has nothing in common with Harris.
Comparing her to Churchill is laughable as well.
Churchill was elected as Conservative MP for Oldham in 1900, before defecting to the Liberal Party in 1904 and spending the next decade climbing the ranks of the Liberal government. He was First Lord of the Admiralty (the civil/political head of the Royal Navy) by the time of the disastrous Gallipoli campaign, which he created. Heavily criticised for this error, he resigned from this position and travelled to the Western Front to fight himself.
The interwar years saw Churchill again ‘cross the floor’ from the Liberals, back to the Conservative Party. He served as Chancellor of the Exchequer from 1924, when he controversially opted for Britain to re-join the Gold Standard. Following the Tory electoral defeat in 1929, Churchill lost his seat and spent much of the next 11 years out of office, mainly writing and making speeches. Although he was alone in his firm opposition to Indian Independence, his warnings against the Appeasement of Nazi Germany were proven correct when the Second World War broke out in 1939.
Besides that he wrote like 10+ important history books.
He is a political comback story for sure, but to compare his pre-prime minister career to Harris?
Roosevelt is maybe a slightly closer comparison maybe, but he was still elected governor of New York. He also won the presidency, while Kamala's attempt at the presidency didn't even make to to the first primary.
There is nothing wrong with hoping or even believing she will turn out awesome. But telling people not to believe what they've already seen is going to be pretty fruitless.