No one deserves to be a president any more than anyone else, and treating an incumbent as though they do, without having to go through an open, democratic primary process, is to treat them as more deserving of future authority than other citizens.
There was a primary, and Biden got the most votes/delegates under the rules. Nobody is saying that incumbents should automatically get renomination. Or even that the incumbent should get some sort of rules advantage (like say, the way the defending world champ in chess gets an auto-bid to defend his title against a challenger who has to win a tournament to get there).
The rules are already set up to where any challenger has an equal structural change of winning the primary. They just won’t have the actual popular support. You know, the core principles of democratic elections.
Same as during a non-incumbent primary. The person who won the most recent nomination tends to have an outsized voice in the selection of party officials (because it’s their pledged delegates who vote on all the other stuff). Yes, that means Biden-affiliated insiders had an inside track in 2020, but that’s also true of Clinton allies in 2016, Obama allies in 2012, Obama allies in 2008, and Kerry allies in 2004.
Which of those changes in the rules do you think were designed to benefit Biden specifically? De-emphasizing the role of Iowa and New Hampshire? There’s been people clamoring for that for decades, within the party.
There’s basically no set of rules that will ever create a credible challenge to an incumbent who wants to run for reelection. It’s a popularity problem, not a structural problem.