I think you are putting too much emphasis on the "digital" part of the lens' DG designation. It seems to be more to differentiate them from "digital" lenses that are APS-C only. Sigma calls their current APS-C only lenses "DC". When digital SLRs first began to gain a foothold in the market, they almost all had sensors that were APS-C or similar sized. So new lenses designed to work with APS-C cameras that could be smaller, lighter, and cheaper than their film counterparts were often referred to as "digital" lenses. With the later introduction and adoption of more Full Frame camera options, and some of them much more affordable than in the past, "digital" no longer means APS-C or smaller as it once did.

Although not universally the case, most lenses designed and introduced during the digital age are better than their older film era counterparts, especially in the consumer and mid grade sectors. Manufacturers of the top tier lenses have also been forced to introduce newer versions of old classics. The new consumer lenses may not be as good as the old "L" glass (but sometimes they get close), but they are much better than yesterdays consumer lenses. Especially zoom lenses which have benefited tremendously from computer aided design and modeling. What used to take weeks or even months to test by making a physical prototype can now be accomplished in a few hours using supercomputer simulation.

Users of digital cameras tend to expect more out of their lenses due to primarily two factors:

• Digital sensors are perfectly flat. Film isn't. Some of the most expensive film cameras actually had mechanisms that created a vacuum behind the film to aid it in laying as flat as possible while being exposed. Even then, with color film the emulsion layer for each color was at a slightly different depth. So if focus was perfect for one color, it would be slightly off for the other two!
• Pixel peeping has raised expectations to a ridiculous level. Take a 20MP image and display it at 100% (1 pixel per screen pixel) on an ≈23 inch HD (1920x1080) monitor and the magnification is equivalent to printing at 56x37 inches! No one expected a 35mm consumer grade lens to be perfect at 56x37! But a lot of folks now seem to.

Using older film era consumer grade lenses on newer digital cameras usually means a bit of a performance hit compared to the newer consumer grade digital era lenses. My Tamron SP 17-50mm f/2.8 Di II, introduced in 2006 as an upper consumer grade "digital lens", compares very favorably to my slightly older design Canon EF 17-40mm f/4 L that was introduced in 2003 during a time when most non-professional photo enthusiasts were still using film SLRs. Go back a little further to the EF28-70mm f/3.5-4.5 (which yields roughly the same FoV with a 35mm camera that 17-50mm does with an APS-C camera) introduced in 1987 and the image quality difference, as well as the speed difference, is quite striking!

On the other hand, in most cases using newer digital era lenses with film cameras will not decrease the performance of the film camera compared to using the lenses' older film era counterparts. But your film camera (and the specific film you are shooting in it) may not be able to take advantage of the additional performance you are paying for with newer digital era lenses. If you use very fine grain film (high quality, low ASA speed) and are printing at rather large sizes or scanning the negatives or slides at very high resolutions it might be worth the difference to you. If you are only printing 4x6 or 5x7s, it might not. Of course, if you have both a digital and film camera in the same mount then the "digital" lenses you use on your DSLR will also work as well or better with your film camera than any older lens designs from the same grade of lenses in their respective eras.