Sadly GL_ARB_texture_compression is something different than GL_EXT_texture_compression_s3tc. The first one only "tells" that texture compression is supported when it's present and that compressed textures can be used. But all my textures are stored using DDS (DXT1, DXT3 and DXT5) and GL_EXT_texture_compression_s3tc is necessary to directly upload these compressed formats via OpenGL. And I thought this was an industry standard, as even valve's OpenGL games need this extension to run. So no, it's sadly not easily possible to move away from that extension.