Look at it this way: Imagine that from Front to Eglinton you could take the subway, between Eglinton and Sheppard you transferred on to an LRT line, then at Sheppard you transferred back on to a subway to Finch, and then at Finch on to the Viva to head north. For those travelling along Sheppard, this will be comparable to that scenario.
Obviously a line must end and cannot go on forever, but there needs to be some sense behind it. Ending the subway at Yonge or Don Mills does not make sense.
And I agree that just using LRT would be best. Convert the tunnel to use LRVs, or convert the rolling stock on the line to be able to run on the street.