You have hit on the basic issue with SIMPLEs. There is no practical penalty for missing the 60 day notice period for plan termination. You have to give an annual notice before 60 days about the ability to make an election. Suppose you do that on October 1, 2018 for 2019. Suppose you decide to terminate the plan for 2019 on 12/15/18 and you notify the employees at that point. OK, you are late according to the IRS website, but where is that in the laws. What is the penalty?
There is a $50 a day penalty under IRC 6693(c)(1) for every day you are late providing a notice, but it is the notice regarding making contributions that is referenced. And even that can be eliminated with "reasonable cause" (but first you have to get caught - how does the IRS ever know about it?). But there is no reference to this "termination notice" except on the IRS website; it is not in the code, and the penalties don't apply to it.
So, in our situation above, there is no penalty on the 2018 year (all is ok) and there are no contributions in 2019 to be penalized ("lose tax benefits"). Is the new 401(k) plan "in trouble". Nope. Does anyone think they can find an actual penalty for providing the termination notice late? I suggest it is really a penalty without a penalty! Which means, maybe you do have until 12/31 to "terminate" the SIMPLE!