By the time he was replaced the institution that he had crafted was pretty much set up, as was the mechanism for rapid expansion of the Heer once it was decided to throw off the WWI peace treaty. The army he left behind was probably the most forward thinking one in the world at that time. This was not only due to von Seeckt, but also the very talented men he had around him who were given a lot of leeway to create the best army in the world. They also created the Luftwaffe, by the way.
I don't think "replaced" was meant in terms of a successor so much as some else being head of the Reichswehr from the start. I think the basic idea that Germany should prepare for rearmament and future war was widely held, so much of what von Seeckt did would have happened regardless. One question might be whether someone else would have been as willing to cooperate with the Bolsheviks.
Ok, I misunderstood. I'm sure there would have been rearmament no matter what, but Seeckt pushed for an army that was totally modern and free from the "we've always done it that way!" mentality. He surrounded himself with the best and brightest officers and gave them free reign to think outside the box, as we say now. He did this while keeping everything under wraps. I'm not convinced that if somebody else had been in the job the Heer would have been the quality that it was in 1939.
It almost certainly wouldn't have been the same. Wether better or worse is hard to say especially without looking at his likely replacements.
Here are some possibilities: Ludendorf, Jochim von Stulpnagel, Kurt von Schleicher and Walter von Brauchitsch.
I think his influence on the Luftwaffe was minimal, on the other hand the core of modern thinkers he encouraged amongst the officer corps greatly contributed to the German victories. I'm not very knowledgeable on the possible alternatives but Ludendorf is likely to go political. This opens the possibility of a slightly less proficient German military but with no Nazi on top as the Army blocked them, that combination could possibly win WW2 for Germany as the drastic resizing imposed by Versailles was going to get rid of the "old school" no matter who was on top.
Actually TOS, the Truppenamt under Seeckt encouraged the growth an air service by including several former aviators. Also included in the second level of officers were Kesselring, Student, Weaver, Jeschonnik and Felmy, among others (many general staff trained) who became important in the Luftwaffe. Also under Seeckt, the secret deal with the USSR was completed which allowed for some hands-on experience when the Germans weren't supposed to even be thinking about an air force. I would also suggest that the Truppenamt could have gone the other way, as in many militaries, and have the "old school" retain control. If that had happened I suspect that Hitler's "Thousand Year Reich" would have had a much shorter existence than it actually had.
While there was an influx of professional soldiers my impression is that in the Luftwaffe the"seat of the pants" former WW1 fighter pilots usually had the upper hand over the professional soldiers. That doesn't look like Seeckt's plan though the background work that made the rapid expansion possible was his doing. So the LW ended up more "old school" than the Heer and that led to a lack of in depth strategic planning (not that the Germans were very good at long range planning but the Heer in general made much less waste of resources than the LW did) . The Germans missed a lot of opportunities because of that.
I think Goring had a few of the "old eagles" around him but I don't think they were anything more than aides. The only "old eagle" in the LW high command, other than Goring himself, was Ernst Udet. Udet, who honestly protested his appointment on the grounds that he wasn't competent for the position, was put in charge of R&D of the LW. Both Goring and Udet were disasters in their respective positions but other, more competent officers, compensated to a degree for their errors. As you probably already know, both Udet and Jeschonnek committed suicide due to the pressures of their office.
Keller,Lörzer,von Richthoven,von Greim,von Pohl were highly decorated pilots in WWI,but also professional soldiers .
I don't think it's a given that it was the "Best" army in the world. While we're giving him credit for the positives should we also give him the responsibility for the German military's comparative lack of supporting forces? Lot's of teeth not enough tail so to speak.
1. Goring and Udet WERE disasters! This was especially true for Goring after the war started. Let's see he was lazy, overly political, a drug addict, more into stealing goodies from the occupied countries than doing his job. His command "style" relied heavily on calling names. As far as I can tell, most other heads of their countries air services were professional military men. Goring was a professional criminal. Udet was a wonderful pilot, womanizer and drunk. He was in no way up to the technical responsibilities of the post given to him. Aircraft development was horribly screwed up under him. However, to his credit, he realized this and tried to get out of the job but Goring insisted he take it. Another check mark against "Fatty"! 2. If the German army wasn't the best in the world in 1939-41, which one was?