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You could definitely ride it as is, my crusty old brompton has been ridden to death with the bridge rotted through and fallen out.
I would fill it with rust inhibitor and plug the hole to stop anything getting in there and making it worse.
That hole won’t affect the rigidity of the frame and if it gets worse you will have plenty of warning. If it does get worse I would maybe replace it later down the line. A fix at this point if done tidily and correctly may just sort it out and go no further.
Getting it replaced would mean taking it somewhere to be fixed, waiting, getting it sprayed etc and I personally wouldn’t want to wait around and potentially end up paying more than the frame is worth.
That looks bad. I would get the bridge replaced before it spreads to the stays.