IGBTs with 3,000+ Volt breakdown specs is revolutionizing the switchover to HVDC. The previous economic breakeven was for a 600 km (360 mi) distance. The US Department of Energy is actively pursuing dropping that to 300 km (180 mi). AC transmission is done in ~70 mi (112 km) segments, at least in North America. Once HVDC cost gets low enough to be competitive at that distance, it will hit a tipping point and become a no-brainer. With about 5 years of transmission-planning backlog and a 30-year design life, it will probably be 2050 before we see the migration mostly accomplished. Of course in the mean time the entire load base, driven by power semiconductors, is natively DC already. One hundred years from now we'll probably chuckle that we ever used AC.
Finally, my understanding is that no one is still using 6 device conversion as shown above and that 12 device is probably the minimum.