I didn't think particularly highly of Starmer and his allies before this whole debacle, but now it's kinda hard to not see them as either completely inept or desperately grasping at power.
Rather than take the fairly obvious boost of simply letting him run as a popular politician and believing the leader they have chosen is good enough to beat a challenger, they have demonstrated they don't believe Starmer to be the leader the membership (and perhaps the rest of the country) would pick.
For an ostensibly democratically run party, disenfranchising your members is a pretty bad look.