I had a similar situation happen years ago, the IRS eventually got in touch with a plan trustee for a late Form 5500 Filing(s) and penalties. Suddenly it was a priority to get the final 5500 done! The trustee was a former owner and anyone that actually worked on the plan was long gone and not answering their phones so he was left on the hook with the plan that he mostly ignored until the letter came.
They were able to avoid the penalties but not without a lot of aggravation, pleading and added legal fees paid by the trustee, in addition to the fees to prepare the late filings.