Given how successful the original Indiana Jones trilogy was and continues to be with the public.. I doubt any iteration that comes next with a new lead (playing Indy or otherwise) will be able to match the long term success and place in culture of those films.
I find this a bit of a navel-gazing statement as well. I don't doubt that there are a bunch of enthusiastic fans, even a sizable one, to whom it may matter whether it's Ford or someone else portraying Indy. But ultimately, these fans make up only a fraction of the movie's targeted audience, and it would be a folly to project those emotions on that wider audience. What ultimately would matter the most to casual viewers is not whether it's Ford portraying Indy or not, but simply whether it's a good film or not. Especially since Disney hasn't yet tried to milk the IP for everything it's worth, like they did with Star Wars.
I'd say you partially got it right with your assessment for why Solo flopped. The main reason for why the audience at large had no strong desire to see a young Han Solo film was because there was too much Star Wars content in the market already - with even more in the way. Solo in 2018 was the fourth annual Star Wars theatrical release in four years' time (after TFA in 2015, Rogue One in 2016 and TLJ in 2017), so even most of the casual audience figured they'll just skip this one and wait 'til they can catch it on Netflix or whatever. If it had been the first Star Wars film released since EPIII or released in 2016 in the slot of Rogue One, it would have had much better odds of making a profit.