The platform you should choose is the one that works best for your business. There are plenty of Magento 2 stores out there. There are also plenty of Bigcommerce and Shopify stores.

It sounds like the real problem is your developer is no longer interested in Magento 2 and would prefer to not have clients using it. This means you either need to find a new developer or keep your anti-Magento developer and risk a decline in quality work. You can also change to Bigcommerce or Shopify, but that can potentially be an expensive option if you don't have a good reason for doing so.