All true, but a big victory at Ypres, especially if they knock Belgium out of the war takes a lot of pressure off on the Western Front. This should allow the Germans to focus on Russia in 1915 and look to launching an offensive on the Western Front in 1916. Perhaps the German High Seas Fleet begins an aggressive campaign in the English channel to take advantage of their improved tactical situation there.
Maybe with some butterflies. But in reality, it makes its worse in most TL. Gallipoli was a mistake. In late 1915, the Entente almost broke the German lines in the West. Well, at least as close as they would come until mid 1918 to a large advance. The Germans had to move 300+ battalion back in a rush move to save the day. If you add in the 15 or so divisions at Gallipoli to France, the Germans commanders lives become a lot harder. Less pressure is applied to Russia. There is an off chance that the Entente win in 1915. It is about the only quick win POD possible for the Entente that does not involve another country joining the Entente at the beginning. I am pretty sure that in most ATL with a "big" German win at second Ypres, you endup with the Ottomans in a vastly better situation and the Germans having more troubles. Probably net win for the Germans, if they survive 1915 ok, since the Ottomans will eventually cause issues elsewhere. Germany having more reserves available in Ypres on the first day of the battle does not increase the overall level of German reserves for the war. My TL is a bit different because the Germans had more reserves for almost the entire war due to the POD.
If you look at what the Entente did, the had a unfocused strategy. Russia pursued a A-H first plan. France a German first plan. UK did Ottomans first plan. If they would have just pick one country, the war goes better for the Entente.
Now if the Germans take Calais, I don't see High Seas Fleet entering the Channel. Too easy to get cutoff. What I do see is a focus on U-boats, mines, and smaller surface ships interdicting the England to France supply routes from Calais to Normandy. Around Normandy somewhere, the "Dover" barrage will be built. What this means is that supplies have to be landed in French ports farther from the front and moved longer distance over rail. It has an important but hard to quantify help for Germans. It is largely the point of my TL how fewer supplies harm the Entente. I can't show exactly which battle it will be, but there will be some battle that was a French win that becomes a German draw. Or a German failed attack becomes a minor breakthrough. Important but not huge. I had the Germans take Ypres and Verdun by the end of 1916.
Now if you have a major butterfly like holding Calais, it will change the German naval decision in many ways. Maybe no ASW. Maybe at different date. Maybe earlier or later type Jutland battle. It is just High Seas fleet entering English Channel is very unlikely. More likely direct actions is UK is concerned about it, so it would move another squadron or two to London or Portsmouth. Maybe from Scapa Flow, more likely ships used at Gallipoli.
And what would I guess a German "big win" means at end of day. Most powers enter war within 6 weeks of OTL dates. Gallipoli cancelled. Ottomans do a lot better. They have enough reserves in east to stop any Russian breakthroughs. Iraq battles fought nearer to Basra. Gaza never falls. We see a lot more Ottoman troops on Eastern Front. Fewer Germans. Germans still lose, but it could be a few months later. Ottomans/Turkey is a lot bigger than OTL. Now I see some potential big butterflies that can help CP win, but each is less than 40% chance of happening. Maybe no USW or not enough USW for USA to enter. Maybe Bulgaria comes in months early or Italy stays out. Maybe UK makes the odd decision to still deal with Gallipoli despite threat to Calais. Odd, Odd call, but possible.