It's obviously your opinion that modern art doesn't belong there, but of course others disagree. The fourth plinth is being used for modern works (selected by a committee, not an individual)on a rotating basis for a reason. It is being reserved for a statue of the queen on horseback, but that will not appear until after her death.
When the statue is in place it will be fulfilling the purpose for which the plinth was originally intended - it was placed there in 1841 for a statue of King William IV on horseback, but that never materialised. People find it hard to believe, but the plinth was actually empty for 150 years. I must have driven around the square hundreds of times, but I never noticed the empty plinth.