Yes Schoenberg step into atonality could be argued as a logical progression and end point from the loosening of tonality throughout the romantic era
However the point I was making was that Schoenberg is a horrible and grossly inadequate romantic composer in contrast to those composer you mention. (as well as other modernist)
The tonal ambiguities matches the expressive intent. For example Wagner stretching of the tonal system matches the sexual tension of the operatic characters of Tristan and Isolde.
Schoenberg piano concerto music does not matches his programmatic and expressive intent of the piece as it is impossible to express the ideas he want to express without using the tonal system. He essentially tried to fit a square in a round hole. If he was adequate composer he would have used tonality in some sections and atonality in other sections.
The irony is that a Robert Greenberg loves Schoenberg but believes the ultraserialist of Boulez, Xenakis, Babbitt and Co went too far because Schoenberg used classical form, romantic expression, phrase structure, timbre, dynamics, rhythm etc to give coherence to Schoenberg works while the ultraserialist abandoned any attempt of coherence at all. Although I do believed that Schoenberg Music is more listenable than the ultraserialist. However I canít criticised the ultraserialist as they are being completely adequate. The ultraserialist believed that Music should predominantly be analyse by the score instead of listened to. That what the piece sounds like is incidental to the point of the music. That the music should be devoid of emotional expression as a reaction against romanticism that they believe contributed to the horror of the two world wars. For what itís worth the ultra serialist were adequate composer who succeeded in their artistic aims even if their artistic aims is awful.