It’s not that simple. Even if getting into earth orbit gets cheaper and less risky, planetary exploration will still be hard, because it is hard and getting into space is not the hardest part.
The relative costs give you an idea of what parts of the mission are hard. A Falcon Heavy launch costs somewhere on the order of $100 million. For a $4.9 billion program, it is not the driver.
Yes, lower launch costs have enabled a lot of missions and cheaper spacecraft than would have been possible otherwise. But space is still hard. There is also an element of the fallacy where if there is more resources available the expectation is cost will go down, but the response ends up people just use more of the resources (there is some economics fallacy/paradox term for this that escapes me right now). Larger launch vehicles enable larger more complex spacecraft. I expect that the capability of the Falcon heavy means that the size of the Uranus spacecraft will be much larger and more complex than something like Voyager 1/2 or New Horizons.
Besides just cost, the engineering constraints can be lower by making designs less complex and easier to do, also, faster. Free delta-v or smaller size constraints, as well as more predictible pricing. Or, 2 for the price of one, or, split up instruments on multiple spacecraft. Reusable spacecraft are a little like that :)
Some of the high mission cost is over-engineering to guarantee success given the already high costs. OTOH the public doesn't want to see NASA failures even at lower costs so there's that...
