fortunately for the population of the planet, scientists and drug companies have not adopted your approach. In the past week, three COVID vaccine candidates entered Phase I trials, and one COVID vaccine candidate entered a Phase I/II trial. If you expand the timeframe to two weeks, the numbers are four candidates entered Phase I trials, one candidate entered Phase II trials, and two candidates entered Phase I/II trials. And if you expand the timeframe to 16 days, you have the numbers from the prior sentence plus one candidate entering Phase III trials.
On a micro level, the Pfizer and the Moderna vaccine temperature requirements remain a big problem for their use in many areas of the world. The J&J vaccine doesn't have those issues. The AZ vaccine also doesn't have those issues, but there is reluctance by a lot of countries to use it, or in populations within countries where it is adopted to get vaccinated with it, due to the blood clot issues that raised in a small number of recipients.
On a macro level, there is a need for a number of vaccine options to reach herd immunity around the world. The data on all of the approved vaccines is only available for less than one year, so it remains to be seen if the vaccines you mentioned provide permanent immunity. Plus, variants are a natural occurrence in vaccines, so even if the existing vaccines provide long-term immunity against the wild and the currently existing variants, in time that may not be the case.
Also, as mentioned in the article, some people have more confidence in a vaccine produced within their nation than in a foreign vaccine. If there are a plethora of options available from a variety of countries, and an issue like that is what it takes many folks to get vaccinated, then the people of the planet are better off for having a wide arrange of vaccine options.