2005-2008 is what I was looking at in which he did not play the field at all, although he was essentially the DH from 1998 on, only (like you say) playing 131 games at 1B the rest of his career. I picked 2005 because he played 0 games at 1b after that year. However, his defensive stats were 'fine' over that period (1998-2004). Maybe moving to DH extended his career in terms of injuries, but I don't really see evidence of that. If the DH doesn't exist, I speculate Frank is still putting up the same hitting numbers and continuing to play 1b, his Hall of Fame legacy intact.
I'd rather use David Ortiz as an example. That guy shouldn't be in the Hall for, primarily steroids, but also the fact that he could not play the field. Frank played four fewer seasons but compiled 6000 more innings in the field. Principally, I think baseball players should play both sides of the ball and I imagine Thomas, Thome, Edgar Martinez could have still played the field and had Hall of Fame careers if the DH never existed. And if they were forced into an earlier retirement, then maybe they weren't good enough at the entire sport of baseball. The sport isn't exclusively hitting and pitching. Frank Thomas is one of my favorite players and the homer in me says he would've made it no matter what. But I sorta think if you can't play the field anymore, if your bad defense outweighs your good offense, it's time to call it quits.
I don't think the MLB ever really needed to make rules to make offense more prevalent (besides the dead ball). Baseball is a pretty slow sport, adding a couple more hits per game isn't going to make it entertaining for would-be fans who think the game is too slow. They can just watch cricket, there's plenty of hitting in that sport and consequently 'bowling', the other side of the ball, is pretty boring; there isn't as much depth as with baseball pitching. Pitchers hitting is also just fun to watch, kind of like comic relief, and there's an extra dimension of strategy when he is expected to hit. A pitcher hitting a home run is far more memorable than a DH hitting one. Bartolo Colon hitting a home run is an awesome moment for the sport, Eloy Jimenez grounding out to the shortstop 90% of the time is...not.
I thought the system was fine when the AL had the DH and the NL did not. Standardization is boring and the relative lack of it in baseball is another aspect of the sport which makes it more interesting than, say, basketball or hockey. Watch how in 20 years every ballpark is going to have the exact same dimensions and feature a dome and I blame Ron Blomberg for it.