My first reaction on seeing clips of the hearing was that Mueller was old and doddery and probably senile.

I have since revised that view slightly.

This was a man who did not want to be there — and probably believed quite strongly that he shouldn’t have been there. But the Democrats had subpoenaed him — not a great way to motivate your supposed allies. Hence his minimalist responses to Democrat grand-standing. They lobbed him juicy little slow-ball questions — in the hope that he would smack them right out of the park — and got next-to-nothing ion return for their efforts. Hence also, his negligible push-back against the at-times brutal Republican onslaught.

He did not want to be there — and he shouldn’t have been there. The reasons why he shouldn’t have been there, were clearly indicated in his opening statement i.e. there are other on-going investigations.

There is a much bigger picture — of which his investigation was only one part. And these other investigations could affect the validity (or otherwise) of his findings.

These other investigations include:
a) The motivations and origins of the known-to-be-false Steele Dossier. Mueller quite correctly (but frustratingly) ‘passed’ on all questions related to that — and it looked bad;
b) The motivations, allegiances and possible guilt of Josef Mifsud. Again, Mueller quite correctly ‘passed’ on all questions related to him — but looked ‘evasive’ and ‘dissembling’ in doing so.
c) The activities and indictment of several Russian personnel of the Russia-based “Internet Research Agency” (IRA). In this case the presiding U.S. District Judge Friedrich found that their indictment contained no allegation — let alone evidence or proof — of any connection between this click-bait outfit and the Russian government. In effect, she warned the AG’s office (and hence Mueller) that if they tried to convey the prejudicial impression that the Concorde/IRA indictment alleged any connection between IRA and the Russian government, they could be held in contempt of court.
d) etc. etc.

Obviously all these other investigations could run-and-run. But the Democrats are in a hurry. The 2020 elections are fast approaching, and they must think Mueller will magically provide them with the ammo they need to hurl at Trump in order to defeat him. (Do the Democrats lack the courage of their convictions, and fear that their policies alone will not be enough to defeat him?)

So they rushed in — and rushed Mueller in. If it has not turned-out well, they have only themselves to blame.