The US could, and DID, make a bewildering variety of Sherman designs and send them to the ETO. Look at the AFV link I posted above; note the number of different suspensions, gun types, engine types... The US also could, and also did, send M-26's to Europe.
Yes, they could only have sent (according to one source I've seen) only one M-26 instead of three M-4's. Which would you rather have: five burned out M-4's with 25 dead crewmen to get one M-4 on the front line, or one knocked-out M-26 with 3 dead crewmen to get one M-26 on the front line with two of the old, and thus experienced, crew?
On average, one crewman was killed when a Sherman was knocked out.*
In 1954, the US Army's Ballistics Research Laboratory conducted a study of tank vs tank engagements fought by the 3rd and 4th Armored Divisions from August to December 1944.
98 engagements were identified, including 33 from the Ardennes fighting. The typical engagement involved 9 US Shermans against 4 German AFVs. Only 1/3 of the total involved more then 3 German AFVs. The average range Shermans inflicted kills on the panzers was 893yds, and the panzers averaged kills at 946yds.
The study concluded that the most important factor was spotting and shooting first. Defenders fired first 84% of all engagement, inflicting 4.3 times more casualties on the attackers then suffered. When the attackers fired first, they inflicted 3.6 times as many casualties on the defenders compared to own losses.
29 engagements involved Panthers and Shermans. The Shermans had an average numerical advantage of 1.2:1. The data showed the Panther had a 10% advantage over the attacking Sherman when the Panther defended, but the Sherman was a whopping 8.4 times more effective then attacking Panthers when the Sherman defended. Overall, the Sherman was 3.6 times as effective as the Panther in all engagements. German A/T guns however, were by far the most effective anti-Sherman weapon they had.
From the study itself:
Data on World War II
Tank Engagements
Involving the U.S.
Third and Fourth Armored Divisions
According to Table II, the most common type of engagement was Shermans defending against Panthers, and the Shermans fired first. In 19 engagements, involving 104 Shermans and 93 Panthers, 5 Shermans were destroyed compared to 57 Panthers.
The second most common engagement was US Tank destroyers defending against Panthers, with the TDs firing first. In 11 engagements, involving 61 TDs and 19 Panthers, 1 TD was lost compared to all 19 Panthers.
The most successful enemy weapon was antitank guns defending. In 9 engagements (3rd most common), 19 a/t guns inflicted 25 casualties on 104 total attacking Shermans, losing 3 guns in exchange.
The 4th most common engagement was Shermans attacking Panthers, and the Shermans fired first. In 5 actions a total of 41 Shermans fought 17 Panthers, losing 2 and taking 12 Panthers in return.
In 40 actions in which the US forces were attacking, they had 437 weapons and lost 100 (23%). The Germans had 135 and lost 45 (33%). In 37 actions in which the Germans were attacking, the US had 205 weapons, losing 14 (7%), and the Germans lost 83 of 138 (60%).
* For comparison, when a T-34 was knocked out, on average, one crewman survived.