The problem isn't just higher frequencies, which as you say getting to optical frequencies will be really hard, but not impossible. The rest of the problem is bandwidth. I can't find the source article for this, but I'd bet the bandwidth is tiny. Visible light covers and entire octave of bandwidth. I've seen zero sign that metamaterials will ever have close to that much bandwidth. Increasing the frequency just means making things smaller. Most of the metamaterials have elements that resonate at specific frequencies.