Why build new ships that can project the kind of firepower a Bengal has, when you don't need such firepower? More to the point, why waste money on such luxuries when you don't even have proper defences against small-scale threats?
Lemme put it this way. Suppose the BW has twenty systems (they have more, IIRC, but I can't be bothered checking). Each of these systems has a jump point into Kilrathi territory. There is no full-scale war with the Kilrathi, but there is a lot of raiding going on. Needless to say, the Border Worlds does not have twenty carriers. Therefore, they have a problem - while even a single escort carrier on station might be enough to deter or drive off a Kilrathi raiding party, they have less carriers than they have systems. In a situation where multiple raids occur simultaneously, the government would have to actually sacrifice some systems. It is clear, then, that what the BW needs now is not heavy firepower, but a bigger fleet of lighter ships. Nobody is questioning that heavy firepower is a good thing to have just in case - but if this firepower needs to be sacrificed in order to ensure a reasonable level of defences for the union, then that's precisely what should be done.
And remember - one of the reasons the BW systems left Confed in the first place was because they felt that Confed neglected their defences. If the union now neglects the defence of some of its systems, what do you think will happen?
. Try to think about this situation from the point of view of the Border Worlds just before the Nephilim show up in Kilrah. Where is this potential huge invasion gonna come from? Will the taxpayers really want to spend money preparing for something that, even an hour before it happened, had something like a 0.00000000000000000001% probability of happening? Would you be happy if your government, right now, started spending billions on defences against a potential UFO invasion?
I could see where it would be benificial for the BW to cut costs any way possible. Still why build new ships that can't project the kind of firepower that a Bengal carrier can. Even today the US government spends billions on defence on the ships that are already in service. I think there are a couple of new ships currently being built but it's not a huge investment as building new ships to replace current vessels that are perfectly capable of carrying out duties. Oh well I've never been able to understand governemntal spending so I don't really see things from their veiwpoint. 
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