The main problem is that any warship, in any era, is geometrically more difficult to replace than, say, ground warfare assets. It's a massive capital investment. You can compare the production time and cost for, say, a Gearing class destroyer to an equal number of Sherman tanks (both 1940's era designs and among the more produced examples in each range.)
THAT is the difference between replacing a Warship (or even a jumpship) and replacing 'mechs. Not just the materials costs, but the lead time needed, if Kerensky started with the listed assets and began building ships right away he likely wouldn't have been able to replace more than a handful of the lighter models (Corvettes, maybe Destroyers)Say, three or four, by the time he called for Exodus...and that's if he started in 2767.
Bigger ships....would be an even bigger problem. the issue isn't so much how long it takes to assemble them, it's how long, and how much infrastructure you actually need to assemble and have going before you've laid the keel.
Over the bulk of the conflict, Kerensky didn't have access to the main production yards, and taking the ones from Amaris by definition disrupted or destroyed teh supply lines necessary to actually USE them for anything more than repairs.
(and not all necessary repairs at that).
A big part of the reason for the Warship Extinction of the 1st and 2nd succession wars, was that supply chain disruption-and the fact it never really recovered in time to let business resume between conflicts. The supply needs to get 'mechs, ASF, or even dropships are tiny comparitively, and in the environments of the Amaris coup and Succession wars, those goods would be higher priority by definition, thus leaving a situation where fleets reduced, but didn't manage to replace-first because of ordinary disruption, then because of the loss of key sections of your naval supply chain via apocalyptic bombardments.
in Kerensky's era, this was going on as well-both sides had to deny the other the abiiity to recover from losses, and both sides had to scramble to keep their supply chains at all.
but that job was a hell of a lot easier for the smaller, non-capital assets. (Dropships, fighters, 'mechs, tanks) so that's where the focus and effort went-because a gun you can't use until tomorrow is worthless if your opponent has guns he can use today...and you don't.