Perhaps the simplest argument against baptism of desire:
There is only one way into the Church: The Sacrament of Holy Baptism.
The Magisterium QUASHED baptism of desire and blood!
Saints and Church Fathers on the absolute necessity of water baptism
Trent's Decree on Justification and Baptism of Desire
The Roman Catechism does NOT teach Baptism of Desire
What Must You Do To Get To Heaven?