EDIT: Had a think, presumably although not wide enough for two lanes of road traffic, the Connel bridge was wide enough for one and the Cregan bridge wasn't? It certainly was a surprise the first time I drove FW to Oban and discovered it! I was living in Leeds so hadn't heard about it.
There must be another reason the bridge was built. The road at Falls of Lora is on the original bridge, is single track and has traffic lights.
BTW the bridge at Cregan saves a long slow trip around the top of Loch Creran which was especially entertaining if there was a very high tide as the normal road floods and the alternative has to be used. Terrific gardens there as well if anyone is of a horticultural bent.
I walked a bike across the Creagan bridge in 1996. Getting onto the south end was a tricky scramble up the embankment, while the north end was more straightforward. It was showing signs of 30 years of disuse, and didn't look capable of taking road traffic.
When I next went to to the area in 2002, the road bridge had been built and I nearly didn't recognise the place. It made a huge difference to travel from the Appin coast towards Oban. I can't see any particular reason to replace the bridge at Connel if it's still in decent nick - and I doubt that the short section of single carriageway is a major traffic issue in the area. On the other hand, as it would more than double the distance between Connel and Ballachulish if you had to divert via Glencoe, the case for replacement would then be quite robust.
As you say, the alternative was a drive round the head of the sea loch, which is 6 miles further round on single carriageway roads (I didn't know about the tidal stuff). The A828's not the best, and that was by the worst section.