In Hinduism Brahmin class maintain strict vegetarian diet forever. What about Islam? Is it sin to maintain vegetarian forever?
I think it is sinful to be vegetarian forever.
O you who have believed, do not prohibit the good things which Allah has made lawful to you and do not transgress. Indeed, Allah does not like transgressors.
In tafseers it says that context of this was abstinence from meat:
Wahidi - Asbab Al-Nuzul by Al-Wahidi
Ibn 'Abbas who related that a man went to see the Prophet, Allah bless him and give him peace, and said: “I have forbidden meat on myself because whenever I eat it I feel the urge to sleep with women”, and so the verse (O ye who believe! Forbid not the good things which Allah hath made lawful for you) and also (Eat of that which Allah hath bestowed on you as food lawful and good…) [5:88].
Kathir - Ibn Al Kathir
It is recorded in the Two Sahihs that `A'ishah said that some of the Companions asked the wives of the Prophet about the acts of worship that he performed in private. One of them said, "I will not eat meat,'' another said, "I will not marry women,'' while the third said, "I will not sleep on the bed.'' When the Prophet heard this statement, he said, «مَا بَالُ أَقْوَامٍ يَقُولُ أَحَدُهُمْ كَذَا وَكَذَا، لكِنِّي أَصُومُ وَأُفْطِرُ، وَأَنَامُ وَأَقُومُ، وَآكُلُ اللَّحْمَ، وَأَتَزَوَّجُ النِّسَاءَ، فَمَنْ رَغِبَ عَنْ سُنَّتِي فَلَيْسَ مِنِّي» (What is the matter with some people who said such and such I fast and break the fast, sleep and wake to stand to pray, eat meat, and marry women. He who is not pleased with my Sunnah is not of me.)
There is no obligation to eat meat in Islam. Hence, there's no tendency in Islam to eat vegetarian. With the resources available in Arabia, there has been little choice, nourishing without animals was practically impossible. In India and with the possibilities of worldwide trade, it is possible. Sacrifice of animals on Eid-ul-Adha is not compulsory for everyone, either.