Would Siddhartha Gautama have agreed with modern forms of Buddhism? Siddhartha Gautama accepted as its close disciples some secular.
They had the same chance to gain enlightenment, without practicing rituals, and without believing in everything he said or did.

Looking at the big picture can show that Siddhartha's truth doesn't hide behind idealism or practicing specific rituals. By it's nature it is everywhere, and it's findable in everything.
The Truth could be in any ritual or practice.

So isn't it out of context to create massive religious groups like we can see in modern Buddhism ?

Isn't it against the nature itself of the truth, to mass share a specific view of it ?
There is a related statement in SN16.13, where the Buddha warns about the decline of his teachings, the true Dhamma, and the rise of counterfeit Dhamma. But at the end, he also gives some hints as to the way to preserve the true Dhamma:


  "... When beings are in decline, and the true
  Dhamma is waning, then there are more precepts and fewer monks attain
  enlightenment. But there is no disappearance of the true Dhamma,
  Kassapa, till a counterfeit Dhamma arises in the world; but when a
  counterfeit Dhamma arises, then there is a disappearance of the true
  Dhamma, just as there is no disappearing of gold so long as no
  counterfeit gold has arisen in the world... The earth-element does not
  make the true Dhamma disappear, the water-element... the
  fire-element... the air-element..., But right here men of straw
  appear, and it is they who bring about the disappearance of the true
  Dhamma. A ship, Kassapa, may sink all at once, but it is not thus that
  the true Dhamma disappears. There are five contributory factors,
  Kassapa, which lead to the adulteration and disappearance of the true
  Dhamma. Which five? It is when monks and nuns, male and female
  lay-followers behave disrespectfully and rebelliously towards the
  Teacher,... towards the Dhamma,... towards the Sangha,... towards the
  training,... towards meditation.
  
  "But when monks and nuns, male and female lay followers behave
  respectfully and deferentially towards the Dhamma,... towards the
  Sangha,... towards the training,... towards meditation, then these
  five things conduce to the maintenance, the purity and the
  preservation of the true Dhamma."


In DN16, the Buddha also commented on the various conditions, if observed, would lead to the growth of the congregation of monks and not its decline. But these are too long to be quoted here. I'll quote only the first set here:


  "The growth of the bhikkhus is to be expected, not their decline,
  bhikkhus, so long as they assemble frequently and in large numbers;
  meet and disperse peacefully and attend to the affairs of the Sangha
  in concord; so long as they appoint no new rules, and do not abolish
  the existing ones, but proceed in accordance with the code of training
  (Vinaya) laid down; so long as they show respect, honor, esteem, and
  veneration towards the elder bhikkhus, those of long standing, long
  gone forth, the fathers and leaders of the Sangha, and think it
  worthwhile to listen to them; so long as they do not come under the
  power of the craving that leads to fresh becoming; so long as they
  cherish the forest depths for their dwellings; so long as they
  establish themselves in mindfulness, so that virtuous brethren of the
  Order who have not come yet might do so, and those already come might
  live in peace; so long, bhikkhus, as these seven conditions leading to
  welfare endure among the bhikkhus and the bhikkhus are known for it,
  their growth is to be expected, not their decline.


Hence, it is not the size of the modern Sangha or congregation of Buddhists that is the problem, but rather it is whether they really follow the teachings or not.

The book "The Broken Buddha" by Ven. Shravasti Dhammika provides good information on how parts of the Sangha and the Buddhist World today may not be reflecting the original ideals put forward by the Buddha.