Those same people will be legally allowed to enter Ireland as of right
So the only people a hard border in ROI/NI would stop is legal EU citizens, choosing to give up all their rights to housing, banking, health care, legal jobs, benefits etc all so they can live in slum housing to work for slave wages and at the risk of prison / deportation if caught.
So which legal EU citizens would want to do this? Which landlord / employer would risk doing this? Yes there are some who do currently but they would be illegal immigrants, not anyone who would be affected by a hard border.
But the UK has to have a hard border with the EU if it wants to enter trade agreements with other countries No it doesn't, just because you insist on one does not mean there has to be one. It would be a political choice by the EU if one went up. We can have a FTA with the EU, we can have zero tariffs with the WTO, we can bill large firms for any taxes electronically, we can have exemptions for small business, we can use all sorts of ways of dealing with different taxes to the ROI (like we do already), there is no necessity to chose a hard border to do it.
So the only arguments for a hard border seem to be: 1) As an excuse to keep us in the EU. 2) To stop people smuggling, even though it couldn't. 3) And to pay taxes (which we could choose not to have) because we live in the 19th century and the only way to pay taxes is by posting a guard to collect cash.
Not good enough arguments.