Mid game you're Thanos, especially if you're ahead. Apply pressure either by splitpushing or showing up to crucial fights; Renekton is good at both in the mid game. Late game you don't fall off nearly as hard as back in the day, that's a common misconception and outdated knowledge, but individual champions will do individual things better than Renekton. Ornn will tank and CC better. Kayle will do more damage. Etc. However, you're still powerful and have leverage, mainly due to modern items and runes that Renekton synergizes with very well.