Could it be that by preventing the natural spread of this virus now and letting many build up immunities?

Yes, but we'll have more ventilators by then (hopefully). It could also be that we'll have a vaccine or a more effective treatment.

DOESN'T MATTER

It would have returned, regardless.

The idea is to reduce deaths now as much as possible, and, to create a vaccine by the autumn.

Yes. There will be a round 2

Building up immunity is what vaccines are for, and they are currently working on developing one.

Preventing the natural spread of the virus now buys us time to find treatments before autumn.  Trump's favorite, chloroquine, should be out of clinical trials by then.  The protein folding folks have a handful of molecules of known safety that can be tried as well.  Buying time is quite valuable.

  In the case of H1N1, we our 1st case was on 04-11-2009.  By 04-26-2009, Obama's HHS had declared Public Health Emergency and re-started vaccine production.  Six mo later, with another emergency to give vaccine, we were better prepared for H1N1 than for most flu seasons.

Some countries in the Far East are experiencing a second wave of Coronavirus as they try to re-open their economies. It could well return, but countries need to be smart about how long they keep lockdowns in place and how quickly they re-open. This is why testing is so important. Hopefully we will have some form of treatment/vaccine in the not too distant future.