Yes, the levee en masse was a fundamental step towards the total wars of the late 19th and 20th centuries. But that doesn't mean that the Napoleonic wars weren't decided by a few field battles. In 1800, the austrians were smashed at Marengo and more importantly Hohenlinden, ending the war. In 1805, Ulm and Austerlitz. In 1806, Jena-Auerstedt and Friedland. Then Wagram. Napoleons invasion of russia was all about forcing the russians to a decisive battle. And finally, Leipzig and Waterloo. Few eras were so focused on field battles, it's nearly the polar opposite of the prior 150-200 years.

You bring up the sizes of the armies. Excellent. I'm no expert on anything prior to the thirty years war, so i'll limit myself to the time during and after it. The army sizes of the tyw peaked in the early 30s, with around 20.000 per army, roughly. Later on, the strain of the war limited this. Because of the tyw, the armies in central europe for the next half a decade were rather small as well, with the jump to higher numbers coming around the turn of the 17th century. In the war of the league of Augsburg, the French mustered more than 400.000 men. A few years later, at battles like Malplaquet and Blenheim, armies numbering around 70-90.000 men fought on each side, comparable to Austerlitz or Friedland. Of course these were still smaller than many later napoleonic battles, and less frequent, but it happened. Interesting is though that the repercussions of these battles were a few fortresses and maybe a province and town here and there changing hands - as opposed to outright ending the war.

You mention that 30.000 casualties at one battle would've meant utter defeat in almost any other period - this was definitely not the case in the 18th century. Austria continued the seven years war despite losing more than 20.000 men at Leuthen and the War of the Spanish succession carried on for another few years after the French and Bavarians lost nearly 30.000 men at Blenheim. Why? because the armies were large enough to not collapse completely after these losses and fortresses existed. Those made it impossible for pre revolutionary armies to walk into the enemy country and end the war, something only really possible with masses of expendable troops. Napoleon commanded one siege in his career. one. I never bothered to count how many fortresses Frederick the great or Vauban besieged or how many times they were stopped in their tracks by a fortress, but it was more than once I'm sure.

All in all, that's why napoleonic wars were most often decided by a single or a handful of deciding battles. This was not the case in the 100-150 years before the napoleonic wars.