No reason why it can't continue on until the original end date but whether or not it's value for that only you can decide.
If it's a decreasing plan then the payout value is going down every year in line with how the original mortgage would have gone so just because you have 30k outstanding now after additional payments doesn't mean the amount that would be paid out on a decreasing policy is the same, it would not have taken into account any of the additional payments.
If it was a level plan as in the amount insured stayed constant throughout the term of mortgage, not as common but not unheard of, then you have a better level of cover but again you might be able to replace that cover with cheaper at this stage if you need it.
Probably best to check exactly what type of cover you have and get a quote for replacement before you decide which will be best.