I think the quality of Delta-sigma DAC chips had gotten very good and will continue to subtly improve, but there is more to a really good DAC than these chips. Quality clocks, output stages, power supplies, enclosures, and even connectors, are quite expensive and not getting any less so. All these things contribute to the sound quality (some more than others).
If anything, I suspect that the sound quality from inexpensive DACs (under $2500) has probably nearly reached its peak since the other parts necessary to build a good DAC are only getting more expensive.