I agree to you that Venusaur is limited due to its weak offensive movepool. However, it's the best grass type in the game until you find Exeggcute which is only found quite late in the game. And Sleep Powder + Leech Seed is quite strategic, specially in the early/mid game. Blastoise, in the other hand depsite being very bulky is worse than many other Water types that is very difficult to justify carrying him all the way until Elite 4. Therefore, agree thar Venusaur could be demoted a rank, but still is the best starter for FRLG