Thanks for raising these issues... This is a major concern for me... Sustainable tourism is a term much abused and the issues appear to a great extent intractable given the conflicting nature of the interested parties most interestingly WHS who appear to want people to look but not touch... I think things only retain there relevance to people because they are used in some way... William Morris (admittedly by way of Tony Wilson) once said: "Nothing useless can be truly beautiful" I believe The Ridgeway and the sites along its route to have beauty and majesty, I hope to awaken an interest in walking mindfully along the route whilst acknowledging and to a great extent incorporating the conflicted nature of the exercise... I know its difficult and has challenges but to the individual and the landscape, but to some extent that is the point of taking a "pilgrimage"?
As an aside I don't think it will mean a great mass of additional people walking the route...
Edited:
Re modern walking pilgrimages what time of year is likely? Are they seasoned or inexperienced walkers? Do they know how quickly the weather can turn on the Downs? There is a lot to consider in this area. Some works I would probably look up would include books and pamphlets by Ken Watts and Adam Stout, both know the monuments very well, the former knows the Downs and routes like the back of his hand, and Adam has actually undertaken a long distance pilgrimage and did so in part study in relation to a historical theme.