someone was upset that this bridge didn't have something that postdated its construction, I pointed out how it was unreasonable. adding them after is a completely separate issue.
on adding the pylons maybe maybe not. bridges, especially long span bridges like that, are highly highly specialized pieces. like maybe a dozen firms world wide that regularly do that type of design work, yet alone construction.
a couple limiting factors would be:
1. the required width of the waterway.
2. erosion/silt build up studies. adding structures would either increase erosion, and risk washing out the structure, or cause unsafe silt build up which would push the bridge, and or restrict the water way faster. either could be designed around, but it wouldn't be as simple as dropping some sheet metal and filling with riff raff.
3. Structurally changing the moments on those bridges could have HUGE impacts. currently/before the accident those columns are "held" down at wherever the foundations are. The columns and the bridge spans would have been designed to move around those points. and yes those bridges need to move. if you build up a structure intended to protect those columns you are moving up where those columns are pinned in place. adding those protections would very likely change the location where loads are transferred from the column. I would imagine raising that point makes the bridge a LOT more rigid, which could be really really bad, unless you redesign the whole bridge.
4. Complete guess because I have never designed a bridge, but I am imagining they have to factor the weight and movement of the water around those columns into the design. you suddenly take that away via a rigid protection and you have likely completely changed the loading on the bridge structure. you may think "less loading on the bridge, less problems". but I would imagine the bridges are built to "lean into" (figuratively not literally) the weight of the water. you take that resistance away and the bridge could tend to push itself out of whack. again it can be designed around, but its not a simple or straight forward process.