If you actually know how to use your plugins, then no, you can make really good sounds with just your stock plugins, especially chaining FX after your synth/virtual instrument
If you are a presets only kinda person, then you will need a LOT of plugins and preset banks to keep your tracks fresh. Honestly, I think it's a very lazy approach to your music. There's nothing wrong with using a preset, but only using presets because you can't make your own sounds is a pretty poor approach to electronic music.
FL studio has imo the best set of stock plugins there are, you definitely can make 99.5% of all sounds using just FL stock plugins and free ones.
It's much more likely that you need to practice using your plugins more rather than buying new ones. If I had the choice, I'd much rather have the FL stock plugs than nexus and sylenth.