If there is not enough cheap manual labor for harvesting crops (i.e. underpaid/exploited illegal aliens), agriculture will just end up more heavily investing in automation, to the extent it makes economic sense. If it does not make economic sense, the capital will be directed elsewhere where it does make sense.

Maybe A.I.

Capital will flow towards where the capitalists believe the profits can be found.

Some crops may not be economical to grow without cheap illegal alien labor, and the economics might not sense to invest heavily in automation to replace the cheap illegal labor. Those products will not be grown here or perhaps only in a very "niche" way.

Perhaps unused farmland will be sold to A.I. companies for data center construction.

Perhaps creative capitalists and inventors will figure out new ways to automate that will be profitable on the farm for certain types of products.

Look at the soybeans, we have so many soybeans that if we can't sell to China the market crashes.