S Corporation Modernization Act of 2019 This bill modifies the tax treatment of S corporations (pass thru entities) to increase from 25% to 60% of S corporation gross receipts the threshold for taxing S corporations with passive investment income; eliminate a provision terminating the status of S corporations with excessive passive investment income for three consecutive years; permit S corporations to have individual retirement accounts as shareholders; allow an adjustment to the basis of an S corporation's assets upon the death of a shareholder, in the form of a 15-year amortization deduction; and permit the Internal Revenue Service to treat a late revocation of S corporation status as timely if it finds there was reasonable cause for failure to make a timely revocation.