Don't need domed stadium, team has learned to use the heat to their advantage. Work the counts, make the opposing pitchers work, watch them start to wilt, Rangers win games if their pitching holds up, which it has come to do. Holland, Harrison, Colby, Feliz, all those guys are acclimated to it and know how to work in it. Yu learned, just as he learns everything. Rangers had more injuries to pitchers this year than anytime in recent memory, and they still kept going ok. It was in the spring, not the Summer, that they had the series with Seattle where they let Seattle score touchdowns instead of runs. I still have nightmares about those games...
No, it wasn't the heat itself that wore them out this year, it was the decision that giving rookies too many spot starts to give the regulars rest in August and, especially, September, was not the best choice given the ongoing dynamics of the division race. In retrospect, that was probably not the correct decision for both physical and mental reasons, but as hot as this summer was, last summer was more hotter more longer, and the team went to the WS for the second year in a row.
Besides, The Ballpark as is is a glorious place to experience a game. Doming it would just suck, period.