I run a small biz, I do PAYE on a simple spreadsheet, then type the results onto an IRD web page, takes me less than 10 minutes a month.
Most people though just use a tax preparer at something like H&R block. A little searching indicates there's roughly 60k to 80k jobs like this in the US, and they earn (at least at H&R Block) about $16 per hour, and only for about 3 months a year. A small # of tax preparers with a bit more training & experience may provide other financial services in the off-season or assist small business with quaterly filings or bookkeeping.
Overall though, you're not talking about much of an industry collapse in terms of people employed. The ones that work in that area year-round will lose some extra hours. The ones that don't, having some experience in finance, might seek further training to get a higher level job of that sort (maybe become an actual accountant) I don't think it should be considered a barrier to ending regulatory capture within this area.
Apart from that, a few software products would collapse, losing most customers, retaining the ones that legitimately needed advanced guidance. And the company would probably have to charge the more for it, and possible compete with actual accountants on price in that respect.
But I don't think "industry collapse" is correct.
Here in the Netherlands I don't have to plug in any data, everything is pre-filled. Income, bank accounts, mortgage, advances on deductions, etc. All you have to do is verify it (you can change things if necessary, but I've never had to do that). Takes about five minutes max.