The Stratis mainchain will never ned to achieve more tps than it already does, which is a maximum throughput of around 60tps (between 60tps and 70tps, depending on which figures you use to calculate it).

All of the heavy lifting is taken by the sidechains. In this sense, the whole Stratis ecosystem is indefinitely scalable: you can always create another sidechain, if the throughput demands are not being met. So the simple answer to your question is yes, and it's already capable of this if so desired. Sidechains allow the ecosystem to be dynamic, responding to the needs of its usership. If the currently achievable throughput isn't enough, then a new sidechain can be created to suit those increased needs.

The Stratis mainchain is pretty much just for carrying out cross-chain transactions. In this sense, the Stratis mainchain is like the currency layer, layer 1. Sidechains are layer 2 tech, and this is the scaling layer. The mainchain layer will never need to scale in the same way. This is because as those cross-chain transactions increase in volume, so too will they increase in size. As the use of the overall Stratis ecosystem increases, the most economically sensible way of performing cross-chain transactions will be with transactions of increasing size. This may be driven by a few popular applications, or even custodial solutions built on top of sidechains.