"Just to be clear, you're saying it's the SNP's fault that the Conservatives and Labour are terrible parties?"
Nope. Strange that this is your take though.
Labour imploded for a myriad of reasons, and that implosion meant there wasn't any effective 'checks & balances' on the SNP over reach. The SNP didn't cause that implosion but certainly benefited from it.
The Tories haven't been a serious political force in Scotland for a long time, but have a disproportionate influence due to being part of the Westminster influence of a Tory majority government. But tory MSPs do little to impact devolved areas. Although that's likely to change in the future (this is an interesting thread on how Westminster will use the internal market bill to usurp devolved administrations & devolved areas where they can - twitter link )
Good government needs a strong opposition to ensure there's not any over reach or bad legislation pushed through without proper scrutiny. Look at the HCB. It's got bad law poorly scrutinised written all over it, and it's getting closer to passing because of the way it's being rushed through to pass before the end of the current session of Parliament. The issues raised& the volume of widespread criticism should have been enough to make them step back, discuss, properly address the many problems that were flagged, and start the Bill from scratch even if it meant carrying over to after the election. But they haven't done that. That's why strong opposition is important - the lack of any opposition being able to challenge the ability of the SNP to push this through is a real problem IMO.