Could it be that these were removed because they are at best 4Mbps because of the state of the art way back when these were designed?
Someone was hired/volunteered to replace this with higher speed gear, but that work was unfinished or being done by someone lacking the proper RF engineering skills for a 3.5mi. bridge link?
I would guess the later.
I would also go with reinstalling it for now and researching/looking for new gear. In theory, if it is working with the antenna and wiring setup now, a newer bridge of roughly similar radio power and amplifier distortion SHOULD be able to manage a faster link rate. That is assuming that the old setup did not have marginal link budget.
However, I'd assume here that it was probably good. 3.5 miles is pretty far, but if it is line of sight with decent antennas on both ends, it isn't really that far. Its just long link in terms of how you must setup the bridge radios to ensure it can actually connect (because otherwise you run in to issues where the bridges are retransmitting packets because they didn't wait long enough to receive acknowledgement from the other bridge).
A decent router to router bridge with 100mw radios and 3dBi omni antennas can easily connect with 65Mbps modulation rate at a distance of 200ft line of sight. 3.5 miles is a LOT further away, but if you assume 4x the transmit power (400mw is a medium powered dedicated bridge), that gets you similar modulation rates at twice the distance (inverse square law), or 400ft.
Moving from 3dBi antennas to 16dBi antennas (medium gain yagi) on both ends is 26dBi higher gain between the two sides. That is roughly 2^4.5 further away, or around 1.8 miles.
This does not take in to ANY kind of diffration, fresenel zone effects, etc., etc.
HOWEVER, I can easily see how you could manage a link at least in the several score of Mbps with a more modern bridge pair at 3.5 miles (I've seen setups where people have managed >20Mbps at distances of >10 miles utilizing 802.11 still and all still being within FCC regulations).
It still may require some tweaking to the current setup, such as mounting the bridges closer to the antennas to reduce coax cable signal attenuation as well as configuration tweaks to the bridges to get them to behave well. You are also likely to see problems in bad weather. a few hundred feet through torential downpours or blizzards isn't likely to change the link strength much at all, even on 5GHz, but over 3.5 miles, atmospheric conditions, from rain to thermals is going to start impacting it quite a bit more, 2.4GHz, 5GHz, 3.6Ghz or 900MHz are all going to start getting skewed by the weather (which doesn't mean it won't work in bad weather, but an ideal day might allow 30Mbps of real through put on the link, but really bad weather might push it down to 5-10Mbps).
At any rate, greater than whatever the link rate was on the old bridges shouldn't be that difficult with some good newer bridges and tweaking the physical configuration as well as the bridge settings.