But without the planetary alignment, can't we just rely on brute force ? Better fuel and bigger engines?

The truth is that, as much as people LOVE bringing it up, the alignment was special only because it allowed us to slingshot from body to body with almost no fuel aboard the probe itself.

That's it. Nothing to do with speed. We could launch something that goes way faster right now, if someone wanted to pay for it. Hell, we could have done it 50 years ago.

We didn't because it would go in a straight line towards "nothing".