Its clear the election was stolen. Should Trump not fight?
It may seem clear, but there's little if any concrete evidence. That's the trouble with a secret ballot: you can't tell who voted for whom (as it should be, for many other reasons), which makes it far more difficult—if not impossible—to tell how fraudulent votes tipped the outcome.
In America, the burden of proof is on the accuser, so with the secret ballot obfuscating the evidence, the accuser is unable to tell what the vote tally was actually supposed to be. So suing is not going to get us anywhere.
The only way to ensure it doesn't happen is through voter security measures that prevent it before it happens. The fact that Democrats oppose such measures should tell you everything you need to know about their motives.
But suing isn't going to do much.