At the moment you just maintain a gas backup infrastructure. Depending on how large your grid is and the intermittency and variation of its sources of electricity, you can assume a certain level of output, then the gap between that and maximum demand needs to be maintained as backup. That’s not necessarily a bad thing, gas plants have a low up front capital cost, with a high marginal fuel cost, so having plants available but used less frequently is not a serious economic barrier.

In the long run energy storage techniques will come in which will displace some use cases for gas, how quickly that happens depends of technological development. As you say long term storage isn’t likely to be achieved by Lithium ion batteries, it’s more likely to be Flow batteries or Hydrogen production. Although Lithium batteries can shift storage throughout a day, and also balance the grid on near instantaneous timescales, which is useful.

Another possibility is overbuilding renewable generation, if solar is very cheap you could produce twice as much as you need in Summer in order to produce enough to meet demand in Winter. That can drastically reduce the need for long term storage, but depends on the properties and cost of the renewable technology in a particular location.