I think the larger challenge is competing with the broad business value that Shopify provides — Its not just payment processing, but also email marketing, theme and API services, a app marketplace, shipping integrations, etc.
Wix/Squarespace, at least last time I checked, treated e-commerce as an add-on to their core service (building websites.) Square is a payment processor which with an e-commerce add-on, but is heavily templated.
BigCommerce is up and coming and basically a "better" version of Shopify. Having managed multiple 7fig stores on Shopify, you're in a constant state of battle against the platform to have it do what you want.
WooCommerce is free plus a $5 a month VPS if you want to do e-commerce yourself. If you want support + hosting + security features then pay for a SaaS.
WooCommerce was purchased by Automattic and since then it's been very difficult to implement without paying for SaaS components. Still possible, but no longer straightforward.
As a small-business Shopify merchant, I would appreciate more competition in the market.