I don’t know why it would be “catastrophic”. Other countries have election season that last barely weeks; there is no reason ours has to go on for years.

It would be catastrophic because a large portion of the base would be enraged that Harris was passed over for someone else.

The problem is that none of the nominees (other than Biden) will have been picked by a primary. If it's not Biden, it's someone who won't be picked by voters. It's someone that will be picked by party leaders and poobahs. Smoke-filled rooms, etc.

That candidate won't have the legitimacy that gets conferred by winning a primary election. They won't have faced the voters. Harris supporters will be outraged that party leaders adjudged her lacking, and there's nothing for the party leaders to point to that's beyond their control. Worse, it's been half a century since either major party picked a candidate at a convention, rather than a primary - so there's no institutional practices or norms that can confer legitimacy onto that choice either. The modern rules were drafted in anticipation of a rubber stamp of the voters' choice, not to be a process that losing candidates can feel resulted in a legitimate contest.

Other countries don't pick their nominees for head of government this way. Nearly all of them are Parliamentary systems, where head of government is chosen by party processes that have a lot of both formal and informal rules to resolve conflicts between different factions in the party. Our parties don't have those things - our methods for resolving conflicts between factions is a contested primary election. The convention can't replace that.