The short answer is no, a MIMO array is much more capable and the IRS will have to be very much larger to achieve similar performance: https://arxiv.org/pdf/2002.04960.pdf

This study only considers SNR, but MIMO arrays also have better multiplexing capabilities and can manage frequency-selective propagation environments.

IRS is rather a competitor to traditional relaying technology. You basically reduce the power consumption at the relay by making it a larger reflector. It is hard to compare the costs before there are any commercial products.

Good question. I had an idea to use a plasma screen which might have some degree of reflectivity. If this is the case then one could design arbitrary array elements that can be turned on and off at will, in a "reflect-array" configuration. Changing the element- and array-pattern might have some adaptive performance. Never tried this out, but I thought that it could be worthwhile to do some simple tests to see if it worked, in principle. In effect this would be a reflective "frequency selective surface" with adaptation.

Yes, it is possible to replace adaptive array antennas with intelligent reflective surfaces (IRS) in wireless communication systems. Intelligent reflective surfaces, also known as reconfigurable intelligent surfaces (RIS), are a relatively new technology that has gained significant interest in recent years due to its potential to improve the performance and efficiency of wireless communication systems.

Intelligent reflective surfaces are passive surfaces that consist of a large number of small elements, each of which can be controlled to reflect or scatter incoming electromagnetic waves in a particular direction. By adjusting the phase and amplitude of the reflected waves from each element, the intelligent reflective surface can create a focused beam or modify the propagation of the signal in a particular direction.

In contrast, adaptive array antennas use multiple antenna elements that are actively adjusted to optimize the reception or transmission of the signal in a particular direction. This requires complex signal processing algorithms and hardware, which can be expensive and power-hungry.

Intelligent reflective surfaces have several advantages over adaptive array antennas. They are passive and do not require active power, making them more energy-efficient. They are also easier to install and maintain, as they do not require any moving parts or complex signal processing hardware. Additionally, intelligent reflective surfaces are capable of reflecting multiple signals simultaneously, which can improve the spectral efficiency of the wireless communication system.

However, there are also some challenges associated with the use of intelligent reflective surfaces. One of the primary challenges is accurately modeling and optimizing the behavior of the reflective surface based on the environment and the characteristics of the signal. Additionally, the performance of intelligent reflective surfaces is highly dependent on the placement and orientation of the surface relative to the transmitter and receiver, which can be difficult to optimize in practice.

In summary, while there are some challenges associated with the use of intelligent reflective surfaces in wireless communication systems, they have the potential to provide a more efficient and cost-effective alternative to traditional adaptive array antennas.