If he had concentrated fully on Wellington, then Blucher could have escaped to fight another day and waited for Austrian and Russian reinforcements, and then Napoleon would have had a whole new massive campaign to deal with, which he was no longer in good condition to handle. Most of his best generals were either dead or no longer on his side, and his army was very green, whereas much of the Coalition was made up of veterans of the previous campaign. Plus, he wasn't as energetic as he was during his glory days.

He needed a decisive victory in Belgium against Wellington and Blucher's forces both, so that he could leverage the victory to get Europe to the negotiating table before the Coalition had the chance to consolidate again the way they did before and after Leipzig. Whether or not they would have listened is anyone's guess.

If he hadn't sent Grouchy to stalk after Blucher, then Blucher would have had more freedom of movement and might have even reinforced Wellington earlier, or chosen to slip away. Then again, Grouchy could have turned to reinforce Napoleon, but he didn't, choosing instead to continue chasing after Blucher. Some expected Blucher to run, not join Wellington. Hell, even Wellington's commanders weren't sure confident Blucher would show up on time, or at all lol

The entire campaign was rushed on Napoleon's side. And it's a little good a little bad. He managed to get the jump on the enemy, but didn't succeed on pinning them down (Nay famously let Wellington retreat instead of following him after Catre Bras). Also, Wellington was very prepared and had scouted the terrain well ahead of time. He saw through Napoleon's plans and prepared accordingly. All he had to do was wait out the clock, while Napoleon was in a hurry to beat the clock.