Without Trump testifying under oath before Mueller:

It would not be possible to establish motivation for collusion.
It would not be possible to prove obstruction of justice
It would not be possible to establish fully that Trump is unfit for office.

So, Trump's lawyers by preventing Trump from testifying to Mueller basically found a way to keep him in office for now.

So now, until Trump is given a subpoena by the Congress or the Supreme Court there will be no way to establish any of the above.

So, though Trump has not been exonerated from any of these charges against him, he has Dodged a bullet by his lawyers keeping him from testifying which would have destroyed him legally since every thing he says tends to be an unnecessary lie.

If Trump had testified to Mueller he would presently be on the road to impeachment. It's not certain that won't happen anyway at this point but this is ONLY the beginning of what comes next.