Can straight samatha ever lead to enlightenment? Can you just do so much samatha meditation that you achieve enlightenment?
If you are asking about whether Samatha meditation alone can lead to enlightenment the answer is no. 

Why is that?

In Vipassana meditation you have ultimate reality as your object of meditation. By having reality as your object you achieve insight into how reality functions i.e. the 3 signs of existence; anicca, dukkha, anatta. When penetrating into reality one can attain enlightenment through realizising one of these signs. 

When one is doing Samatha meditation alone one can only gain tranquility i.e. a strong concentration and the jhanas. One cannot any achieve insights into reality and therefore not become enlightened. That is because in samatha meditation one does not have ultimate reality as an object. Instead one has a concept as an object e.g. one of the 10 kasinas. One is trying to keep that object static by keeping attention on the object through the means of applied and sustained thought. 

You can think about it in another way. Lets say you had a big stone in front you and you want to carve out a figure from that stone. To do that you need an instrument, a tool. Concentration is your tool and the stone is reality.

In order for you to be able to carve into the stone you need the tool to be sharp. In other words you need to sharpen your tool. That is where samatha meditation is used. You use samatha meditation to train and sharpen your concentration so that you can use concentration as a tool to carve/penetrate into reality and then gain insights into how reality functions.

So back to the stone. If you only sharpen your tool will that ever lead you to enlightenment? 

No its not possible. Samatha meditation is like that sharpening. By doing samatha meditation you are only making your tool sharper, better and more precise but you are never actually applying that tool to the stone. You are never actually carving the stone.

That is why samatha meditation alone cannot lead you to enlightenment.

Lanka