The whole reason for N+1 is to bypass ASML's EUV lithography machine. N+1 is in fact slightly inferior to true 7nm process (which requires EUVL), in both performance and power consumption, it's closer to 8nm than of standard 7nm. Why pour time and resources into developing N+1 then?
One obvious reason is to bypass ASML's monopoly on EUVL, in case US pressures them to not sell. But that doesn't explain the fact that SMIC started developing N+1 prior to Huawei ban.
The original, and far more important reason is cost and scalability. EUVL machines are huge, and hugely expensive. Each one needs to have its own crane on site just to be serviced. And servicing equipment fills up an entire building, requires a horde of specialized engineers. This makes wafers produced by it hugely expensive, but also acts as a bottleneck for production volume. Each fab is limited by the number of EUVL machines they can afford to buy and use.
Keep in mind that SMIC actually acquired an EUVL machine in 2018, but did not use it to produce 7nm chips. It was sitting there for "research purposes." (wink, lol) Why didn't they stop bothering with N+1 and just use that machine? Here's why:
By being able to bypass EUVL with N+1, SMIC is able to produce chips that are still slightly inferior to true 7nm, but can be produced at 14nm costs, and without quantity limit or the huge initial investment necessary for buying additional EUVL machines. Google the price gap between 14nm and 7nm, and you'll see. In other words, they will be able to flood the market with chips at a fraction of the price, in quantities that can expand without limits. Standard chips larger than 7nm will have no market. The reason TSMC and other fabs are so desperate in reaching ever smaller processes, is they need to stay ahead of the wave - for example, once SMIC further develops N+1 to reach, or even just come close to 5nm specs, that market segment is done for.
Herein ends your free education. To verify the above, or to learn more, Google is your friend. Like it or not, the Chinese are taking over the fab industry, like they did with many other industries over the years. They will do it by flooding the market with cabbage price chips. History repeats. I can't say it's not a good thing for us consumers though.