Well, the 32nd century of DSC was created before other shows like Prodigy which show other forms of propulsion which makes standard warp drive obsolete. The result of which being that DSC isn't in continuity with these other shows (something that CBS cares extremely little about), and the only rational way to explain it all is that any new space drive technology shown that's superior to standard warp drive must only be temporary or shown to be ultimately faulty, just so continuity can be maintained with season 3 of DSC. Which, as you say, is extremely unrealistic.
Actually, more advanced methods of propulsion and power generation technologies were introduced all the way back in VOY (such as: QS v2, Coaxial Warp, and Transwarp - some even in TNG)... which happened BEFORE Prodigy.
So, the writers had plenty of material to work with already... I'm just guessing they decided to completely ignore it.
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