This is not really true. I help run and launch professional news organizations on WordPress, some of which have tremendous archives of 100s of thousands of posts.
The post meta table can scale fine to hundreds of thousands of post objects without needing special infra or handling. The WordPress core has a caching layer for metadata, and either way it's not like "SELECT * FROM wp_postmeta WHERE meta_key = 'foo' AND post_id = 1234" is going to be a performance bottleneck regardless of the size of the meta table.
I'm not referring to single meta_key look ups but was referring to non-indexed key + value queries on large wp_postmeta tables. Eventually a developer will need to do a meta_key + meta_value look up in some query which will tank performance on large table sizes. With plugins like Advanced Custom Fields, the wp_postmeta table can drastically increase in size and a developer can quickly fire off dozens of non-indexed key + value queries in one request. Given JSON support in MySQL, Postgresql, et al, there is no need to not migrate away from such a poor schema. Source: also work in the media world and have large WP installations in my charge. WP is a resource burden.
The post meta table can scale fine to hundreds of thousands of post objects without needing special infra or handling. The WordPress core has a caching layer for metadata, and either way it's not like "SELECT * FROM wp_postmeta WHERE meta_key = 'foo' AND post_id = 1234" is going to be a performance bottleneck regardless of the size of the meta table.