By that logic, nobody needs to write any thing because there's always some "online service" to "do stuff".

I think the logic is that those served by PHP are better served by platforms. Those with advanced needs are going to use a different language.

PHP isn't being replaced so much by Python, Go, and Rust as it is being replaced by Medium, Wix, and Squarespace.

As someone who works with PHP reasonably often, I'd say you have a very limited view of what PHP is actually used for in 2020, if you think 3 hosted blog services will replace it.

The majority of PHP usages tend to be, frankly, unsophisticated Wordpress, WooCommerce, Magento, and Drupal sites that are hacked up to support e-commerce. These needs can now be met by a vast array of ecom and omnichannel platforms.

This is the direction things are trending.