If a Muslim refuses to perform sacrifices upon the practice or “ethic” of the “veg lifestyle,” then they are willingly abstaining from mandatory religious practices. This is a major problem, as not performing mandatory religious obligations is sinful and punishable.

Loosing major rewards
A Muslim also faces the problem of forgoing some supererogatory acts, and by doing so, forfeit major rewards and blessings that can count in their favour of good deeds.

The Companions of the Messenger of Allah (ﷺ) asked: “‘O Messenger of Allah, what are these sacrifices?’ He said: ‘The Sunnah of your father Ibrahim.’ They said: ‘What is there for us in them, O Messenger of Allah?’ He said: ‘For every hair, one merit.’ They said: ‘What about wool, O Messenger of Allah?’ He said: ‘For every hair of wool, one merit.'” (Sunan Ibn Majah)

Sacrificing animals on Eid al-Adha
This is an established sunnah which is performed annually by Muslims. It is highly recommended to sacrifice ones wealth and time to purchase an animal for the purpose of slaughter, sharing of meat, and consumption. Its performance shows gratitude to Allah in commemoration of the sacrifice of Prophet Ibrahim AS; it is considered honouring the symbols of Allah the Almighty. Allah says: “…And whoever honours the symbols of Allah – indeed, it is from the piety of hearts.” [22: 32] and: “Their meat will not reach Allah, nor will their blood, but what reaches Him is piety from you.” [22: 37].

Children will be held indebted to Allah
The Messenger of Allah (ﷺ) said:
“‏ الْغُلاَمُ مُرْتَهَنٌ بِعَقِيقَتِهِ يُذْبَحُ عَنْهُ يَوْمَ السَّابِعِ وَيُسَمَّى وَيُحْلَقُ رَأْسُهُ ‏”
“The boy is mortgaged by his ‘Aqiqah; slaughtering should be done for him on the seventh day, he should be given a name, and his head should be shaved.” (Sahih) (Jami` at-Tirmidhi 1522)

As long as the slaughter has not been performed for the son or daughter, then the child is held in “bondage” so to speak with Allah. This debt cannot be paid except via slaughter.

‘Aishah (RAA) narrated, “Allah’s Messenger (ﷺ) commanded them to slaughter two sheep equal (in age) for a boy and one for a girl at their birth.’ (Related by At-Tirmidhi and graded it as Sahih)

Allah says in the Quran,
وَلِكُلِّ أُمَّةٍ جَعَلْنَا مَنسَكًا لِّيَذْكُرُوا۟ ٱسْمَ ٱللَّهِ عَلَىٰ مَا رَزَقَهُم مِّنۢ بَهِيمَةِ ٱلْأَنْعَـٰمِ ۗ فَإِلَـٰهُكُمْ إِلَـٰهٌ وَٰحِدٌ فَلَهُۥٓ أَسْلِمُوا۟ ۗ وَبَشِّرِ ٱلْمُخْبِتِينَ
“For every community, We appointed a rite of sacrifice so that they may pronounce the Name of Allah over the sacrificial animals He has provided for them. For your God is only One God, so submit yourselves to Him ˹alone˺. And give good news ˹O Prophet˺ to the humble.” [Al Hajj 22: 34]

Missing out on major rewards
Allah says in the Quran:
لِّيَشْهَدُوا۟ مَنَـٰفِعَ لَهُمْ وَيَذْكُرُوا۟ ٱسْمَ ٱللَّهِ فِىٓ أَيَّامٍ مَّعْلُومَـٰتٍ عَلَىٰ مَا رَزَقَهُم مِّنۢ بَهِيمَةِ ٱلْأَنْعَـٰمِ ۖ فَكُلُوا۟ مِنْهَا وَأَطْعِمُوا۟ ٱلْبَآئِسَ ٱلْفَقِيرَ
so they may obtain the benefits ˹in store˺ for them, and pronounce the Name of Allah on appointed days over the sacrificial animals He has provided for them. So eat from their meat and feed the desperately poor. [Al Hajj 22: 28]

The Prophet (ﷺ) concerning the udhiyah (the sacrificial animal) said, “Eat some, store some and give some in charity.” [Narrated by Muslim 1971].

Missing out on cures and other barakah
Allah says in the Quran that he put a healing for people in honey. For vegans in particular, this is problematic, as honey is derived from insects.

Allah says in another verse, “And your Lord inspired the bees: “Make ˹your˺ homes in the mountains, the trees, and in what people construct, and feed from ˹the flower of˺ any fruit ˹you please˺ and follow the ways your Lord has made easy for you.” From their bellies comes forth liquid of varying colours, in which there is healing for people. Surely in this is a sign for those who reflect.” [Quran 16: 68-69]

Thus Islamically the “Muslim vegan” will be left out of receiving these great benefits.